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Single-cell sequencing

Single-cell sequencing is a collection of advanced biotechnologies that enable the detailed analysis of genomic, transcriptomic, epigenomic, and proteomic profiles from individual cells, uncovering molecular heterogeneity that is obscured in conventional bulk sequencing methods which average data across cell populations. This approach has revolutionized the understanding of cellular diversity, dynamics, and interactions within tissues and organs, facilitating breakthroughs in precision medicine and biological research. The origins of single-cell sequencing trace back to early efforts in , with the first demonstration of whole-transcriptome sequencing (scRNA-seq) achieved by Tang et al. in 2009 using a single blastomere, detecting expression of 5,270 genes—75% more than achieved by contemporary techniques. Building on this, single-cell whole-genome sequencing emerged in 2011 through Navin et al.'s work, which sequenced DNA from individual nuclei in samples to infer tumor and copy number variations. Subsequent innovations in the 2010s, including Smart-seq2 for improved full-length cDNA amplification and droplet-based systems like Drop-seq (2015), scaled throughput to thousands of cells, while multi-omics methods such as (2017) began integrating and protein data. Key technologies in single-cell sequencing span several modalities: for , multiple displacement amplification (MDA) and multiple annealing and looping-based amplification cycles (MALBAC) address whole-genome coverage challenges; transcriptomics relies on scRNA-seq variants like plate-based (e.g., Smart-seq) and high-throughput droplet or microwell methods (e.g., ); epigenomics includes single-cell bisulfite sequencing (scBS-seq) for and assay for transposase-accessible chromatin (scATAC-seq) for chromatin accessibility. Unique molecular identifiers (UMIs) and barcoding enhance accuracy by correcting for biases and enabling multiplexing. Applications of single-cell sequencing are vast and interdisciplinary, including dissecting tumor microenvironments in cancer to identify rare subclones and therapeutic targets, as seen in melanoma and breast cancer studies; mapping immune cell states in infectious diseases like COVID-19; and charting developmental trajectories in embryogenesis and organogenesis. Large-scale efforts, such as the Human Cell Atlas initiative launched in 2016, leverage these technologies to catalog all human cell types across tissues, promoting global collaboration in creating reference maps for health and disease. Despite its advancements, single-cell sequencing faces hurdles like amplification-induced biases, low capture efficiency leading to sparse data, high per-experiment costs, and computational demands for integrating multi-omics datasets and inferring cell trajectories. Ongoing developments in and long-read sequencing promise to address these limitations, further expanding the field's impact.

Introduction

Definition and principles

Single-cell sequencing encompasses a suite of high-throughput next-generation sequencing (NGS) techniques applied to individual cells to profile their genomic, transcriptomic, epigenomic, or multi-omic features, thereby unveiling cellular heterogeneity within complex populations that bulk methods cannot resolve. This approach enables the detection of molecular variations at the single-cell level, such as differences in DNA copy number, expression, or accessibility, which are critical for understanding tissue diversity, developmental processes, and disease states. At its core, single-cell sequencing operates on principles of isolating individual cells from a sample, lysing them to access nucleic acids, amplifying the target molecules while minimizing bias, and incorporating barcodes or unique molecular identifiers (UMIs) to enable and during NGS. These steps allow for the precise quantification of molecular content per cell, highlighting advantages like the identification of rare subpopulations, transient cellular states, and fluctuations that are obscured in population-level averages. For instance, the inaugural single-cell sequencing experiment demonstrated the ability to detect the expression of over 5,000 genes from individual oocytes and blastomeres, exposing expression heterogeneity not detectable in bulk analyses. In contrast to bulk sequencing, which pools nucleic acids from thousands to millions of cells and yields an averaged signal masking intra-population , single-cell sequencing captures cell-to-cell variability, including transcriptional and subclonal . This distinction is exemplified in early single-cell applications, where analysis of individual tumor cell nuclei revealed punctuated clonal expansions and genetic substructures in breast cancers, providing insights into tumor unattainable through bulk tumor profiling. The basic workflow of single-cell sequencing generally proceeds from cell isolation—using techniques such as fluorescence-activated cell sorting or microfluidic encapsulation—to and capture, followed by reverse transcription or amplification, barcoded library preparation, and deep NGS to generate per-cell molecular profiles.

Historical development

The roots of single-cell sequencing lie in early methods for probing at the individual cell level. In the 1990s, single-cell quantitative PCR (qPCR) techniques were developed to quantify the expression of a small number of genes, offering the first glimpses into cellular heterogeneity without the averaging effects of bulk analyses. The introduction of next-generation sequencing (NGS) platforms around 2005, including the 454 sequencer and Illumina's Genome Analyzer, marked a critical enabling step by providing scalable, high-throughput capabilities that would later support single-cell applications. A landmark achievement came in 2009 with the first demonstration of single-cell sequencing (scRNA-seq) by Tang et al., who sequenced the transcriptome of blastomeres using a involving mRNA capture via oligo() priming and in vitro transcription for linear amplification, followed by NGS. This proof-of-concept overcame key technical hurdles in amplifying and sequencing minute amounts of cellular , setting the stage for broader adoption. The 2010s saw rapid innovations in throughput and automation. In 2012, Fluidigm launched the C1 Integrated Fluidic Circuit system, which automated single-cell capture, , and cDNA in microfluidic chips, enabling consistent processing of dozens to hundreds of cells. Droplet-based methods further scaled the technology: in 2015, Macosko et al. introduced Drop-seq, a scalable approach encapsulating cells and barcoded beads in oil droplets for massively parallel scRNA-seq of thousands of cells. That same year, Klein et al. developed inDrop, an improvement using beads for more efficient barcoding and encapsulation, achieving similar high-throughput . In 2016, commercialized the platform, building on these principles to deliver commercial-grade, droplet-based kits. Single-cell DNA sequencing (scDNA-seq) emerged in the early 2010s, primarily through whole-genome amplification techniques like to detect copy number variations (CNVs) in cancer cells, as exemplified by Navin et al.'s 2011 study mapping genomic heterogeneity in breast tumors. In , the 2013 development of single-cell (scBS-seq) by Guo et al. allowed genome-wide profiling from individual cells, revealing epigenetic diversity in early embryos. This was followed in 2015 by Buenrostro et al.'s single-cell for transposase-accessible sequencing (scATAC-seq), which mapped open regions to infer regulatory states at single-cell . Multi-omics integration advanced in the late , with introduced in 2017 by Stoeckius et al. to simultaneously measure transcripts and surface proteins via oligonucleotide-tagged antibodies, enhancing phenotypic resolution. The brought spatial and multi-modal extensions, combining single-cell sequencing with in situ technologies to preserve tissue context. By 2025, state-of-the-art systems routinely profile over 1 million cells per run, with increasing integration of for enhanced data interpretation and noise reduction.

Core Technologies

Cell isolation techniques

Cell isolation is a critical initial step in single-cell sequencing, enabling the physical separation of individual cells from heterogeneous samples such as tissues or cultures to facilitate downstream molecular analysis. Various techniques achieve this by leveraging physical, optical, or biochemical properties, each balancing factors like throughput, cell viability, purity, and preservation of native states. These methods are essential for minimizing contamination from neighboring cells and ensuring high-quality single-cell profiles, particularly in applications like transcriptomics and . Fluorescence-activated cell sorting (FACS) employs fluorescently labeled antibodies to target specific surface markers, using excitation and light scatter detection to sort viable s into collection tubes or plates based on size, granularity, and intensity. This technique achieves high purity exceeding 95% and throughput rates up to s per second, making it suitable for large-scale isolation from dissociated samples, though it can induce due to high-pressure ejection and requires labeling that may alter viability. FACS has been widely adopted for single-cell sequencing since its adaptation for multi-parametric sorting in the 1990s, as demonstrated in early applications for enrichment. Magnetic-activated cell sorting (MACS) utilizes superparamagnetic beads coated with antibodies that bind to surface antigens, allowing separation via a that retains labeled s on a column while unlabeled s pass through. It offers gentle handling with viabilities often above 90%, scalability for 10^6 to 10^8 s per run, and lower cost compared to FACS, though it provides binary (positive/negative) selection with less resolution for multiple markers and potential non-specific binding. MACS is particularly useful for enriching rare populations prior to single-cell sequencing, as shown in protocols for isolating . Microfluidic approaches, including droplet encapsulation, integrate cell partitioning within microchannels or oil-in-water emulsions to isolate thousands to millions of cells in parallel. In droplet-based systems like those used in platforms, cells are co-encapsulated with barcoded beads in nanoliter-scale droplets, enabling automated, high-throughput isolation (up to 10,000 cells per sample) with minimal manual handling and reduced cross-contamination. These methods excel in scalability and integration with and but require uniform cell suspensions and can suffer from droplet instability or doublets. Seminal droplet technologies, such as Drop-seq and inDrop, revolutionized by achieving encapsulation efficiencies of 5-10% for input cells. Laser capture microdissection (LCM) involves mounting tissue sections on a film and using an or UV to selectively cut and catapult specific cells or regions into a collection cap, preserving spatial context from fixed or frozen samples. It provides precise of hundreds of cells per session with minimal dissociation artifacts but is low-throughput and labor-intensive, with risks of RNA degradation in archival tissues. LCM is ideal for heterogeneous tissues like tumors, where it has enabled single-cell genomic profiling since its development in the late 1990s. Manual picking employs micromanipulators or micropipettes under an inverted microscope to aspirate individual cells from a monolayer or suspension, offering high precision for rare or morphologically distinct cells without labels. This method yields near-100% purity but is extremely low-throughput (tens of cells per hour) and operator-dependent, limiting its use to small-scale experiments like validating sequencing results. It remains a gold standard for isolating viable primary cells, as in protocols for neuronal subtypes. Emerging label-free techniques, such as dielectrophoresis (DEP) and acoustic sorting, exploit intrinsic cell properties to avoid antibody-based labeling. DEP uses non-uniform to manipulate cells based on their properties, enabling on-chip of cells with throughputs of hundreds per minute and viabilities over 90%, suitable for sensitive applications like sorting. Acoustic methods apply standing surface to separate cells by size or compressibility in microfluidic channels, achieving high purity (up to 99%) and gentle handling at rates of 100-1,000 cells per second without physical contact. These approaches are gaining traction for scalable, non-invasive in single-cell sequencing workflows.

Library preparation and sequencing platforms

Library preparation for single-cell sequencing begins with the of isolated cells to release nucleic acids, followed by capture of target molecules such as mRNA via poly-A tail selection for transcriptomic . Reverse transcription then converts to cDNA, often incorporating unique molecular identifiers (UMIs) to tag individual transcripts and mitigate amplification biases during subsequent steps. Barcoding is applied at the cellular level using combinatorial indices, enabling multiplexing of thousands of cells in a single sequencing run, while targeted amplification enriches libraries for high-throughput next-generation sequencing (NGS). For single-cell genomic sequencing, whole-genome amplification (WGA) is essential to generate sufficient DNA from limited input, with methods like multiple displacement amplification (MDA) providing uniform coverage through isothermal strand-displacement synthesis, though it can introduce chimeric artifacts, or degenerate oligonucleotide-primed PCR (DOP-PCR) for higher-resolution copy number variant detection via PCR-based enrichment of genome regions. In transcriptomic workflows, cDNA synthesis techniques vary by coverage needs: Smart-seq2 enables full-length transcript capture using template-switching oligo during reverse transcription, ideal for isoform analysis, while CEL-seq focuses on 3'-end tagging for cost-effective gene counting with linear amplification via in vitro transcription to reduce bias. Key platforms for library preparation integrate these steps with automation for scalability. The Chromium system employs with Gel Bead-in-Emulsion (GEM) technology, encapsulating up to 100 million s weekly alongside barcoded beads for UMI and indexing with the Chromium Flex as of 2025, a leading provider with major players collectively holding 70-75% market share. Plate-based approaches like the Fluidigm C1 system process 96 to 800 cells in integrated fluidic circuits with optional for viability confirmation, suitable for lower-throughput experiments. Open-source droplet methods, such as Drop-seq, use simple microfluidic devices to thousands of cells cost-effectively via aqueous nanoliter droplets, while inDrop employs hydrogel-encapsulated primers for similar high-throughput barcoding. Kit-based solutions like Parse Biosciences' Evercode platform enable scalable preparation up to 5 million cells per run without specialized instruments, leveraging combinatorial split-pool barcoding for accessibility in academic settings. Sequencing of these barcoded libraries typically relies on short-read platforms like the Illumina NovaSeq, which delivers high-throughput output (up to 6 Tb per run) for demultiplexing and read in single-cell applications. Emerging long-read technologies, including PacBio's high-fidelity circular sequencing and Nanopore's direct readout, are increasingly adapted for single-cell use to resolve full-length transcripts and isoforms, though they currently offer lower throughput compared to short-read systems.

Single-cell genomic sequencing

Methods

Single-cell genomic sequencing focuses on analyzing the DNA sequence, structure, and variations within individual cells to uncover genetic heterogeneity masked in bulk samples. Key methods rely on whole-genome amplification (WGA) to generate sufficient DNA from the limited input (typically 1-10 pg per cell) for high-throughput sequencing. Early techniques include degenerate oligonucleotide-primed PCR (DOP-PCR), which uses semi-random primers for initial amplification but suffers from uneven coverage. Multiple displacement amplification (MDA) employs phi29 DNA polymerase for isothermal amplification, achieving higher fidelity and ~50% genome coverage at 20× depth, as demonstrated in studies of B lymphocyte mutations. Improved methods like multiple annealing and looping-based amplification cycles (MALBAC) reduce bias by forming DNA loops to prevent over-amplification of early products, enabling better uniformity for (CNV) detection. The first single-cell whole-genome sequencing was reported by Navin et al. in 2011, sequencing DNA from cell nuclei to infer tumor evolution and subclonal CNVs using MDA. Subsequent platforms, such as Fluidigm's C1 system (introduced 2013), automate cell isolation and WGA for scalable processing. High-throughput approaches incorporate barcoding for multiplexing, including the for CNV profiling and Mission Bio's Tapestri for targeted sequencing of mutations in hematologic cancers. Recent advances as of 2025 emphasize long-read technologies to resolve structural variants (SVs) and haplotypes. For instance, single-cell with direct long-read sequencing (dMDA) generates reads up to 10 kb, achieving ~40% coverage with 15.7 Gb of data, while SMOOTH-seq enables genome assembly from ~30 cells with contig N50 lengths of ~1.35 Mb. These methods, using PacBio or Oxford Nanopore platforms, detect 16-fold more SVs than short-read approaches and support phasing with fewer than 100 cells. Single-nucleus isolation is often preferred for solid tissues to avoid DNA damage during dissociation.

Limitations

Single-cell genomic sequencing is challenged by technical biases and inefficiencies inherent to WGA from minute DNA quantities. Amplification bias leads to uneven coverage, with MDA and DOP-PCR showing chimeric artifacts and allelic dropout rates up to 30-50%, where one allele fails to amplify, complicating variant calling and heterozygosity detection. MALBAC mitigates this but still achieves only ~70-90% uniformity compared to bulk sequencing. Coverage sparsity is prevalent, often limited to 10-50% of the per at practical depths (e.g., 0.1-1× average), requiring deep sequencing (15-30 per ) for reliable CNV or detection, which escalates costs to $1-5 per for short-read methods and higher for long-read. Long-read approaches as of 2025 improve for repeats and SVs but face low throughput (<100 s per run) and error rates (1-5% for early Oxford Nanopore), though recent iterations reach >99% accuracy. Computational demands for assembling fragmented, biased data and correcting errors remain high, with phasing feasible only in low-heterozygosity samples. These issues limit applications to targeted panels over full genomes in routine use.

Applications

Single-cell genomic sequencing reveals intratumor genetic diversity and evolutionary dynamics, aiding precision oncology. In , it has mapped CNVs and subclones to trace tumor progression and , identifying therapy-resistant variants. Similarly, in (AML), targeted sequencing tracks clonal evolution under FLT3 inhibitor treatment, informing relapse mechanisms. Beyond cancer, it detects somatic mosaicism in neurodevelopment, sequencing hundreds of neurons to uncover low-frequency mutations linked to and . In infectious diseases, it profiles integration in host genomes, such as HIV proviral loads in immune cells. Large consortia like the Human Cell Atlas incorporate scWGS for baseline genetic maps across tissues. As of 2025, long-read methods enable whole-chromosome phasing in embryos, advancing reproductive genetics and studies in rare diseases. These applications highlight its role in dissecting at cellular .

Single-cell transcriptomic sequencing

Methods

Single-cell transcriptomic sequencing, primarily through single-cell RNA sequencing (scRNA-seq), captures the of individual cells to reveal heterogeneity. Key methods include plate-based approaches, such as Smart-seq2, which amplify full-length cDNA from polyadenylated mRNA using template-switching oligo technology, enabling detailed isoform and allele-specific expression analysis but limited to lower throughput (hundreds of cells). Droplet-based methods, like Drop-seq and the Chromium platform, encapsulate single cells with barcoded beads in oil droplets for high-throughput profiling of thousands to millions of cells, typically capturing 3'-end transcripts with unique molecular identifiers (UMIs) to mitigate amplification biases. Microwell-based systems, such as Microwell-seq, offer cost-effective alternatives by loading and barcoded primers into arrays of sub-nanoliter wells, supporting scalable 3'-end sequencing with reduced reagent use. Combinatorial indexing techniques, including SPLiT-seq, further enhance throughput by applying multiple rounds of barcoding without physical partitioning, enabling profiling of up to 100,000 per experiment as of 2023 advancements. Cell isolation precedes library preparation, often via fluorescence-activated (FACS) or , followed by reverse transcription, amplification, and sequencing on platforms like Illumina NovaSeq. Recent 2025 developments, such as optimized Smart-seq3 variants, improve capture efficiency for low-input samples, achieving near-full-length coverage in diverse tissues.

Limitations

Single-cell transcriptomic sequencing faces challenges related to technical variability and data sparsity. Capture efficiency is typically low, around 10-20% of the per cell, leading to dropout events where low-expressed genes appear undetected, complicating differential expression analysis. Amplification biases from methods like introduce noise, particularly in full-length protocols, while droplet-based approaches suffer from doublets (two cells in one droplet) at rates of 1-5%, requiring computational filtering. The dissociation of tissues for single-cell suspension can alter transcriptomic states, introducing stress responses or losing fragile cell types, and the method inherently discards spatial information, necessitating integration with for context. High costs, estimated at $0.50-2 per cell for large-scale runs as of 2025, and computational demands for processing sparse matrices (often 80-90% zeros) limit accessibility, though open-source tools like Seurat mitigate analysis barriers.

Applications

Single-cell transcriptomic sequencing has transformed biological by enabling high-resolution mapping of types and states. In cancer, scRNA-seq dissects tumor heterogeneity, identifying rare subclones and therapy-resistant populations, as in studies revealing evolutionary trajectories. In , it profiles immune responses, such as T-cell exhaustion in , uncovering dynamic shifts in expression across infection stages. Developmental biology benefits from trajectory inference, tracing lineage decisions in embryogenesis, exemplified by atlases of human highlighting regulatory networks. Neurological applications include charting brain cell diversity, linking transcriptomic signatures to disorders like Alzheimer's via integration with genetic data. As of 2025, large consortia like the Human Cell Atlas incorporate scRNA-seq for comprehensive tissue maps, advancing precision medicine through biomarker discovery.

Single-cell epigenomic sequencing

Methods

Single-cell epigenomic sequencing methods primarily target and chromatin accessibility to reveal epigenetic heterogeneity across individual s. For profiling, single-cell (scBS-seq) involves cell lysis followed by to deaminate unmethylated cytosines, enabling differentiation from methylated ones during subsequent whole-genome and sequencing. This approach, often using post- genome tagging (PGT) for library preparation, typically covers only 1-10% of CpG sites per cell due to inefficiencies in and , limiting genome-wide resolution but allowing detection of methylation patterns in rare cell types. Early implementations demonstrated accurate measurement at up to 48.4% of CpGs in optimized cases, highlighting its utility for assessing epigenetic variability in and . Chromatin accessibility is commonly assayed using single-cell assay for transposase-accessible with sequencing (scATAC-seq), which employs the Tn5 transposase to insert sequencing adapters into open chromatin regions via tagmentation, typically performed on isolated nuclei to preserve structure. Following tagmentation, Nextera-based library preparation incorporates cellular barcodes—often integrated via combinatorial indexing strategies—to enable and demultiplexing of thousands of cells. This method generates 1,000 to 50,000 unique reads per cell, sufficient to identify accessible regulatory elements like promoters and enhancers, though coverage sparsity necessitates computational imputation for deeper insights. scATAC-seq has been pivotal in mapping regulatory landscapes in diverse tissues, revealing cell-type-specific motifs. Variants of these core methods expand profiling capabilities while addressing scalability. For instance, single-cell , , and transcription sequencing (scNMT-seq) combines nucleosome positioning via GpC methyltransferase labeling of accessible regions with conversion for methylation and poly-A capture for transcripts, providing integrated epigenomic views from the same cell, though focused here on its epigenetic components. Similarly, single-cell combinatorial indexing (sci-ATAC-seq) enhances throughput by using split-pool barcoding to profile chromatin accessibility in up to tens of thousands of cells per run, reducing costs and enabling population-scale epigenomic atlases without sacrificing single-cell resolution. These indexed approaches build on nuclei techniques to minimize artifacts and integrate barcoding during preparation for efficient sequencing. Single-nucleus protocols are essential for epigenomic sequencing of solid tissues like , where whole-cell dissociation can introduce biases from incomplete or cytoplasmic contamination, altering perceived accessibility or signals. By isolating intact nuclei, these methods preserve epigenetic marks in post-mortem or frozen samples, facilitating unbiased profiling of neuronal diversity and disease-associated changes. Recent advancements, such as sciMETv3 introduced in 2024, enable atlas-scale single-cell profiling through combinatorial indexing, supporting applications in fixed or archived samples to broaden accessibility for longitudinal studies. This method achieves high-throughput coverage of millions of CpGs across tens of thousands of cells, improving efficiency over earlier bisulfite-based techniques while maintaining compatibility with diverse sample types.

Limitations

Single-cell epigenomic sequencing encounters challenges related to low input material, amplification biases, and data sparsity. For scBS-seq, conversion efficiency is typically 90-95%, but whole-genome amplification introduces dropout and biases toward high-CG regions, resulting in uneven coverage and detection of only 1-20% of CpGs on average, with many cells showing zero calls at low-input sites. These issues limit resolution for global landscapes, particularly in non-CpG contexts, and require deep sequencing (often >100 million reads per cell) to mitigate noise, increasing costs to $10-50 per cell. scATAC-seq suffers from high sparsity, capturing only 1-10% of open regions per due to low fragment recovery (often <5% relative to bulk), leading to 80-90% zero counts in feature matrices and challenges in distinguishing true accessibility from technical noise. Nuclei isolation helps reduce biases in solid tissues but can miss cytoplasmic factors influencing , while multiplexing introduces barcode collisions (up to 10-20% in high-throughput runs). Computational demands for peak calling and imputation are high, as sparse data hampers motif discovery and trajectory inference without advanced denoising. Overall, these methods yield lower throughput than transcriptomics (typically 1,000-10,000 cells per run versus 10,000+), with per-cell costs 2-5 times higher due to specialized reagents. Integration with other modalities remains limited by technical variability, though ongoing improvements in aim to enhance coverage as of 2025.

Applications

Single-cell epigenomic sequencing uncovers epigenetic diversity driving cellular identity and disease, enabling mapping of methylation patterns and accessible regulatory elements at individual-cell resolution. In cancer, and dissect tumor heterogeneity by identifying methylation signatures in rare subclones and chromatin remodeling in therapy-resistant cells; for example, in glioblastoma, revealed enhancer rewiring in glioma stem cells linked to invasion. These insights guide precision oncology by pinpointing epigenetic vulnerabilities for demethylating agents like . In developmental biology, methods like and chart epigenetic trajectories during embryogenesis, such as nucleosome positioning and DNA methylation waves in mouse neural progenitors, illuminating how chromatin states dictate lineage commitment. Applied to human fetal brain tissues, these approaches have cataloged cell-type-specific enhancers associated with neurodevelopmental disorders like , linking variants to regulatory disruptions. In immune responses, single-cell epigenomics profiles dynamic chromatin accessibility in T cell activation, revealing locus-specific opening during differentiation in COVID-19 patients. Large initiatives like the Human Cell Atlas incorporate epigenomic data to create reference maps of tissue-specific methylation, aiding biomarker discovery for autoimmune diseases as of 2025. Recent advances, including sciMETv3, have extended applications to archived biobanks, enabling retrospective studies of epigenetic drift in aging and environmental exposures.

Single-cell multi-omics sequencing

Methods and approaches

Single-cell multi-omics methods enable the simultaneous profiling of multiple molecular layers, such as transcripts, proteins, chromatin states, and genetic perturbations, from individual cells to uncover regulatory mechanisms and cellular heterogeneity. These integrated protocols typically leverage shared barcoding strategies or compartmentalization to link modalities without sacrificing throughput, distinguishing them from unimodal assays by providing direct correlations within the same cell. Paired assays combine transcriptomics with proteomics or epigenomics using antibody-based capture. CITE-seq (Cellular Indexing of Transcriptomes and Epitopes by Sequencing) pairs poly-A mRNA capture with oligo-tagged antibodies for surface proteins, allowing quantitative measurement of both transcriptomes and up to hundreds of epitopes in thousands of cells via droplet-based sequencing. Developed by Stoeckius et al. in 2017, this approach has become a standard for dissecting immune cell states by correlating surface markers with gene expression. Similarly, REAP-seq (RNA Expression and Protein Sequencing) extends this paradigm to include intracellular or epigenetic targets, using barcoded antibodies alongside RNA barcoding to profile proteins and transcripts in parallel, as demonstrated by Peterson et al. in 2017 for analyzing T-cell responses. Split-nucleus approaches physically separate nuclear and cytoplasmic components to access distinct layers while preserving single-cell resolution. scNMT-seq (single-cell Nucleosome, Methylation, and Transcription sequencing), introduced by Clark et al. in 2018, isolates intact nuclei for parallel assay of nucleosome positioning via MNase digestion, DNA methylation by bisulfite conversion, and nuclear transcripts, yielding multi-modal profiles from ~1,000 cells to link epigenetic modifications to transcriptional activity in embryonic stem cells. Building on this, SHARE-seq (Simultaneous High-throughput ATAC and RNA Expression with sequencing), developed by Ma et al. in 2020, employs split-pool combinatorial indexing on permeabilized nuclei to jointly capture open chromatin via and poly-A RNA, enabling scalable profiling of regulatory elements and expression in >10,000 cells per run and revealing chromatin "potential" in lineage priming. Perturb-seq integrates by coupling perturbations with multi-omics readout. This method captures synthetic barcodes alongside single-cell RNA profiles in pooled screens, allowing perturbation effects to be deconvolved at resolution. Pioneered by Dixit et al. in 2016, Perturb-seq screened ~2,000 in immune cells to map transcriptional responses and regulatory circuits, scaling to genome-wide assessments in subsequent iterations. Spatial multi-omics methods embed multi-modal capture in tissue context to map molecular interactions across microenvironments. The Visium HD platform from , launched in 2024 and enhanced in 2025, uses high-density barcoded slides (2 μm resolution) for whole-transcriptome profiling combined with oligo-tagged antibodies for protein detection, supporting unbiased of frozen or FFPE tissues at near-single-cell scale. For epigenomic integration, adaptations of Slide-seq enable spatial co-profiling of accessibility and transcripts; Bao et al.'s 2023 method transfers tissue sections for joint and , achieving ~100 μm resolution to visualize epigenetic landscapes in and tumors. High-throughput platforms streamline multi-omics via commercial droplet systems for massive parallelization. The Chromium Multiome ATAC + assay, introduced in 2020, partitions nuclei into droplets for simultaneous GEX library preparation and ATAC tagmentation, capturing linked and accessibility from up to 10,000 nuclei per sample to infer enhancer-gene connections in diverse tissues. Extensions incorporate reporters like GFP for protein tracking.

Limitations

Single-cell multi-omics sequencing faces significant challenges in correlating multiple data types due to inherent technical discrepancies between . A primary issue is modality mismatch, stemming from varying capture efficiencies; for instance, single-cell RNA sequencing (scRNA-seq) typically captures about 10% of transcripts per cell, while single-cell ATAC sequencing (scATAC-seq) achieves substantially lower efficiency, often recovering less than 5% of accessible fragments relative to bulk methods. This disparity results in 20-50% of cells exhibiting discordant profiles across modalities during , as and signals fail to align reliably, particularly in paired datasets where nuclear isolation for ATAC-seq may alter transcriptomic states. Noise levels are elevated in multi-omics approaches, exacerbating correlation difficulties. In , which combines transcriptomics with surface , cross-reactivity introduces false associations, with nonspecific binding leading to spurious protein-RNA correlations and elevated false discovery rates in up to 10-20% of features. Split-pool methods, used to assay multiple layers sequentially, further compound noise by decoupling measurements from the same cell, losing precise spatiotemporal co-localization and amplifying batch effects between modalities. Throughput reductions are common, as integrating multiple assays demands more complex library preparation and sequencing depth allocation. For example, commercial kits like Multiome process up to 10,000 cells per run for combined scRNA-seq and scATAC-seq, compared to higher capacities in single-modality scRNA-seq with newer platforms. This limitation drives up costs, with multi-omics workflows estimated at ~$0.05-0.25 per cell as of 2025, versus ~$0.01-0.10 for optimized single-omics, due to specialized reagents and extended sequencing needs. Data sparsity is amplified in combined multi-omics profiles, where individual modalities already exhibit high zero counts—often 80-90% in scRNA-seq alone—resulting in over 70% zeros across integrated datasets, hindering downstream correlation and imputation. Incorporating spatial multi-omics exacerbates this by introducing resolution loss; many platforms operate at sub-cellular or 50-100 μm scales, averaging signals from multiple cells and diluting modality-specific details. As of 2025, long-read multi-omics can profile thousands of cells per experiment, as aligning and assembling long reads (e.g., 6-10 kb) across transcriptomic, epigenomic, and genomic layers generates terabytes of data requiring intensive processing resources.

Applications

Single-cell multi-omics sequencing enables a comprehensive dissection of cellular heterogeneity by integrating transcriptomic, epigenomic, and other molecular layers, offering insights into regulatory mechanisms and functional states within complex biological systems that single-modality approaches cannot achieve. This holistic perspective is particularly valuable for elucidating dynamic interactions in heterogeneous environments, such as those involving cell-cell communication, developmental trajectories, and responses to perturbations or environmental stresses. In the , single-cell multi-omics technologies like have revealed intricate immune-tumor interactions, including the role of + M2-like macrophages in promoting immune exclusion and tumor progression. For instance, profiling identified a spatial niche where these macrophages colocalize with stem-like tumor cells, correlating with T cell exclusion and adverse clinical outcomes in various cancers. This integration of surface protein and data highlights immunosuppressive networks that drive therapy resistance, informing targeted interventions to disrupt these interactions. During neurodevelopment, scNMT (single-cell nucleosome, methylation, and transcription) sequencing has mapped gene-regulatory links across cortical layers, providing a multi-layered view of how epigenetic modifications orchestrate -type diversification. Applied to cortical nuclei, scNMT datasets from over 63 types uncovered regulatory diversity, linking distal enhancers and promoters to disease-associated variants in excitatory neurons and . These findings illuminate the epigenetic-transcriptional dynamics underlying laminar organization and vulnerability to neurodevelopmental disorders. In assessing responses, Perturb-seq and its multi-omics extensions, such as Multiome Perturb-seq, evaluate the effects of genetic editing in CAR-T cells by simultaneously capturing perturbation-induced changes in and chromatin accessibility. Genome-wide screens using these methods in CAR-T platforms like CELLFIE have identified enhancers of T cell and antitumor , revealing regulatory circuits that mitigate exhaustion and improve outcomes in hematologic malignancies. This approach deciphers how edits modulate multifunctional states, guiding optimization of next-generation CAR-T therapies. For organoids, the Multiome assay validates tissue mimicry by jointly profiling expression and accessibility, enabling comparison of structures to native tissues. In colorectal tumor organoids, Multiome analysis of matched patient-derived samples distinguished cancer-specific cell states and trajectories absent in healthy organoids, confirming their fidelity in recapitulating tumor heterogeneity and stromal interactions. Such applications underscore organoids' utility as models for testing therapeutic responses while quantifying deviations from architecture. Recent advances in 2025 have applied to coral regeneration under stress, using scRNA-seq to uncover molecular mechanisms in reef ecosystems. In stony corals like Acropora muricata, these methods revealed cellular dynamics enhancing thermotolerance and regeneration post-bleaching.

Data Analysis

Preprocessing and

Preprocessing and in single-cell sequencing involve initial steps to process raw sequencing reads, assess , filter artifacts, and normalize counts to ensure downstream analyses reflect true biological variation rather than . These steps are crucial for handling the high-dimensional, sparse nature of single-cell data, where factors like sequencing depth and can dominate signals. Read processing begins with demultiplexing FASTQ files using cell barcodes and unique molecular identifiers (UMIs) to assign reads to individual cells, followed by error correction to mitigate sequencing inaccuracies. For sequencing, reads are typically aligned to a using spliced aligners like , which accommodates introns and improves mapping accuracy in transcriptomic data. In contexts, such as single-cell , unspliced aligners like BWA are employed for precise genomic alignment. Quality control metrics guide cell identification and filtering. Cell calling in droplet-based methods often uses knee plots to distinguish true cells from empty droplets by plotting barcode rank against UMI counts, typically retaining barcodes above an . Low-quality cells are filtered based on metrics such as detecting fewer than 200-500 s per cell, indicating poor capture efficiency, or exceeding 20% mitochondrial content, which suggests cellular or . Doublets, arising from multiple cells being encapsulated together, are detected and removed using tools like Scrublet, which simulates doublets and scores them above 0.5 for exclusion. Additional filtering targets ambient contamination, where free-floating transcripts from lysed cells inflate counts; methods like SoupX estimate and subtract this background by modeling non-cell-associated profiles. Cells with fewer than 1,000 reads are commonly removed to eliminate those with insufficient coverage. Normalization adjusts for variations in sequencing depth and library size. For scRNA-seq, log-normalization scales counts by total reads per cell, while SCTransform employs a regularized negative model to stabilize variance and account for technical noise more robustly. Counts per million () provides a simple scaling for comparative expression analysis. Batch effects from different runs or samples are corrected using algorithms like , which projects data into a shared low-dimensional space to align distributions without over-correcting biological signals. Popular tools for these steps include Seurat in , which integrates , normalization via SCTransform, and batch correction in a modular , and Scanpy in , offering efficient handling of large datasets with similar functionalities. As of 2025, Alevin-fry, an extension of the quantifier, enables rapid pseudoalignment and UMI deduplication for single-cell data, reducing processing time for massive datasets while maintaining accuracy.

Clustering and interpretation

After preprocessing, single-cell sequencing data undergoes to project high-dimensional profiles into lower-dimensional spaces, facilitating visualization and downstream analysis. () is a linear method that captures the principal axes of variance, serving as an initial step in many workflows due to its computational efficiency and interpretability. Nonlinear techniques like () emphasize local structure preservation for cluster visualization but can distort global relationships. Uniform Manifold Approximation and Projection (UMAP) improves upon t-SNE by better maintaining both local and global structures, often yielding more interpretable embeddings in single-cell datasets. maps, which model data on a manifold, are particularly useful for inferring developmental trajectories by emphasizing temporal progression. Benchmarks across diverse datasets show UMAP and maps outperforming t-SNE in structure preservation, with adjusted Rand index (ARI) scores up to 0.75 for trajectory tasks. Clustering algorithms group cells into populations based on similarity in the reduced space, typically using graph-based approaches that construct a k-nearest neighbors (kNN) graph weighted by expression distances. The Louvain algorithm, implemented in the Seurat R package, optimizes modularity to detect communities, enabling scalable clustering of thousands of cells. The Leiden algorithm, available in the Scanpy Python toolkit, refines Louvain by ensuring better connectivity and resolution, reducing the risk of disconnected clusters. For simpler datasets, k-means clustering partitions cells into a predefined number of groups by minimizing intra-cluster variance, though it assumes spherical clusters and struggles with complex manifolds. The resolution parameter in graph-based methods controls cluster granularity; higher values yield finer partitions, tunable via silhouette scores or domain knowledge to balance over- and under-clustering. Recent benchmarks indicate graph-based methods like Leiden achieve ARI values of 0.80-0.90 on peripheral blood mononuclear cell datasets, outperforming k-means in hierarchical structures. Interpretation of clusters involves identifying biological significance through marker gene detection and functional annotation. The Wilcoxon rank-sum test compares gene expression distributions between clusters and the rest, flagging differentially expressed genes as markers; for instance, CD3D often marks T cells with log-fold changes exceeding 1.5 and adjusted p-values below 0.01. Pathway enrichment analysis, such as (GSEA), assesses coordinated changes in gene sets like pathways, revealing upregulated signatures in specific clusters. Pseudotime inference orders cells along developmental trajectories; Monocle3 learns branched graphs via reversed , while fits principal curves to clusters for linear or diverging paths. These tools correlate pseudotime with gene expression gradients, identifying dynamic regulators like transcription factors in differentiation processes. In multi-omics contexts, integration methods jointly model modalities like and protein data to enhance clustering robustness. MOFA+ performs with group-specific priors, decomposing variation into shared and modality-unique factors for cross-omics alignment. totalVI, a deep , uses to integrate count-based data, accounting for technical noise and enabling probabilistic cell-type assignment. Benchmarks on datasets with , ATAC, and protein profiles show these methods improving integration concordance, with normalized scores around 0.85 compared to unimodal approaches. Advanced tools like scVI leverage variational autoencoders (VAEs) for probabilistic modeling, imputation of dropout events, and batch correction, enhancing clustering accuracy by reconstructing latent spaces.

Applications in Biology and Medicine

Basic biological research

Single-cell sequencing has revolutionized basic biological research by enabling the dissection of cellular heterogeneity and dynamics at unprecedented , revealing the intricate of tissues, developmental trajectories, and evolutionary processes that underpin life. This technology allows researchers to profile individual s across diverse modalities, such as transcriptomics and , providing insights into fundamental mechanisms without the averaging effects of analyses. By capturing the diversity within populations, it facilitates the mapping of architectures, commitments, microbial communities, mutational landscapes, and stress responses in non-model systems like . In tissue heterogeneity studies, single-cell sequencing has been instrumental in creating comprehensive atlases that catalog types and states across organs, highlighting the diversity and shared features among populations. For instance, the Tabula Sapiens atlas profiles nearly 500,000 s from 24 s and organs of multiple donors, identifying organ-specific variations in behaviors while revealing conserved transcriptional programs across types, such as T clonal sharing between immune-related organs. This resource underscores the mosaic-like structure of s, where rare subpopulations and transitional states contribute to overall function, advancing our understanding of multicellular organization. For , single-cell sequencing integrates transcriptomic and epigenomic data to trace lineage specification during key events like , elucidating how cells commit to embryonic fates. In human , single-cell multi-omics approaches, such as scMAT-seq, have mapped epigenetic landscapes alongside , showing how accessibility and dynamically regulate lineage priming in epiblast cells transitioning to , , and progenitors. These profiles reveal principles of epigenetic memory and stochasticity in early development, providing a framework for modeling cell fate decisions. In microbial ecology, single-cell genomics has unlocked the genetic potential of uncultured , which constitute the vast majority of microbial diversity, by isolating and sequencing individual cells from environmental samples. This method bypasses cultivation biases, enabling the recovery of high-quality genomes from previously inaccessible taxa, such as rare gut symbionts or microbes, and revealing metabolic pathways like fiber degradation in human-associated . By profiling single cells, researchers can infer ecological roles and interactions in complex communities, transforming our view of microbial ecosystems. Single-cell sequencing also illuminates evolutionary dynamics within cell populations by quantifying mutation rates and phylogenetic relationships at the individual level, offering a window into neutral and adaptive processes. In somatic tissues, single-cell whole-genome sequencing reconstructs phylogenies that estimate mutation accumulation rates, demonstrating how proliferative dynamics influence genetic diversification independent of disease contexts. These analyses show that mutation rates vary with cell division history, providing quantitative measures of evolutionary tempo in healthy lineages. In plant biology, recent single-cell RNA sequencing efforts have exposed cellular heterogeneity in responses to environmental stresses like , identifying cell-type-specific regulatory networks that drive . A 2025 study on trees used single-nucleus transcriptomics to profile development under , revealing auxin-mediated arrest in cambial cells while other zones maintain , thus highlighting compartment-specific mechanisms.

Clinical and translational applications

Single-cell sequencing has revolutionized clinical and translational applications by enabling precise characterization of cellular heterogeneity in diseases, facilitating earlier diagnosis, targeted therapies, and strategies. In , it plays a pivotal role in dissecting tumor evolution and immune interactions, while in and infectious diseases, it uncovers cellular mechanisms underlying and persistence. These insights have translated into clinical tools for monitoring treatment responses and identifying biomarkers, with ongoing advancements enhancing non-invasive detection methods. In cancer, single-cell DNA sequencing (scDNA-seq) has been instrumental in tracking subclonal evolution and , particularly in hematological malignancies like (AML). For instance, high-throughput scDNA-seq of 123 AML patients revealed intricate clonal architectures and mutational histories, showing how resistant subclones emerge under therapeutic pressure and inform relapse prediction. Similarly, longitudinal single-cell profiling during in AML demonstrated that early treatment failure often stems from pre-existing resistant subclones, guiding adaptive dosing strategies to target these populations. Complementing this, single-cell sequencing (scRNA-seq) identifies immunotherapy targets by mapping tumor-immune interactions; studies have used scRNA-seq to discover biomarkers predicting immune checkpoint blockade (ICB) responses, such as exhausted T-cell signatures in solid tumors, enabling patient stratification for therapies like PD-1 inhibitors. In , single-nucleus RNA sequencing (snRNA-seq) has illuminated neuronal diversity in (AD), revealing cell-type-specific vulnerabilities that drive progression. A comprehensive single-cell transcriptomic atlas of multiple regions from 283 post-mortem AD cases identified regionally distinct neuronal subtypes with altered and tau-related gene expression, highlighting excitatory neuron loss as a key pathological feature and potential therapeutic target. These findings support the development of disease-modifying drugs aimed at preserving specific neuronal populations, bridging basic heterogeneity insights to design. For infectious diseases, single-cell multi-omics combining scDNA-seq and scRNA-seq has mapped viral reservoirs, exemplified by persistence despite antiretroviral therapy. Integrated single-cell epigenetic, transcriptional, and protein profiling of latent -1-infected cells uncovered heterogeneous states in + T cells, including transcriptionally active reservoirs that evade clearance, informing strategies like latency-reversing agents. In transplant , single-cell sequencing monitors immune rejection by profiling T-cell states in allografts; scRNA-seq of renal biopsies delineated clonal + T-cell responses during acute cellular rejection, identifying effector and regulatory subsets that predict graft outcomes and enable tailoring. By 2025, multi-omics integrated into liquid biopsies has advanced early cancer detection with high sensitivity. Multi-omics models combining methylation and protein markers achieved 81.9% sensitivity for stage I-II gynecological cancers at 96.9% specificity, outperforming single-modality approaches.

Challenges and Future Directions

Current technical and computational challenges

Single-cell sequencing faces significant technical hurdles that limit its widespread adoption and accuracy. The high cost of conducting large-scale studies remains a major barrier, with comprehensive single-cell RNA sequencing (scRNA-seq) projects often requiring substantial funding in the millions of dollars depending on scale and modalities, primarily due to expensive reagents, instrumentation, and computational resources required for processing thousands to millions of cells. Cell dissociation from tissues frequently results in significant viability loss during enzymatic or mechanical processing, which introduces biases in transcriptional profiles and reduces the representation of fragile cell types such as neurons or endothelial cells. Additionally, many multi-omics approaches, which integrate transcriptomics with proteomics or epigenomics, suffer from low throughput, often capturing fewer than 10,000 cells per run due to technical constraints in simultaneous molecular capture and amplification, though high-throughput methods are emerging. Computationally, the vast dimensionality of single-cell datasets poses formidable challenges, as matrices comprising approximately 10,000 genes across 1 million cells can generate terabytes of raw data, necessitating advanced storage and processing infrastructure to handle sparsity and noise. Batch effects, arising from variations in experimental runs or protocols, can account for up to 30% of total variance in expression profiles, complicating the integration of datasets from different labs or time points and potentially masking true biological signals. Imputation methods to address dropout events—where genes are undetected due to low mRNA capture efficiency—often introduce inaccuracies; for instance, the Markov Affinity-based Graph Imputation of Cells () algorithm can add around 15% error in recovered expression values, distorting downstream analyses like . Standardization efforts are hindered by the absence of universal benchmarks, leading to issues where (CV) across laboratories exceeds 20% for key metrics such as levels in pseudo-bulk aggregates. Ethical concerns further complicate data handling, particularly risks in large-scale human atlases, where linking attacks on count matrices can re-identify donors with high accuracy from seemingly anonymized public datasets. in annotation also persists, often stemming from reference dataset imbalances that favor abundant types and underrepresent rare or tissue-specific populations, resulting in misclassification rates of 10-30% in heterogeneous samples. As of 2025, the integration of spatial transcriptomics exacerbates data management challenges, with high-resolution imaging platforms generating terabytes of data per tissue slide due to the need to align millions of spatially resolved transcripts with imaging metadata. These volumes strain current computational pipelines, particularly for multi-modal spatial datasets that combine RNA with protein or DNA information.

Emerging technologies and perspectives

Recent advancements in long-read single-cell sequencing technologies, particularly those leveraging PacBio's single-molecule real-time (SMRT) sequencing, have significantly enhanced isoform resolution by capturing full-length transcripts averaging 1-2 kb, enabling the detection of complex events at the single-cell level. For instance, PacBio's Revio platform, with its high-throughput capacity of up to 480 Gb per day across four SMRT cells, supports the processing of approximately 1,000 cell lines in projects like the 1000 Genomes Long Read initiative, which uses Kinnex kits to generate isoform data and address limitations in short-read methods for resolving transcript diversity. These developments, including techniques like HIT-scISO-seq and MAS-ISO-seq, concatenate multiple cDNAs for improved efficiency, paving the way for broader applications in dissecting cellular heterogeneity beyond current short-read constraints. In spatial omics, innovations such as MERFISH combined with sequencing enable high-plex transcriptomics, achieving up to 500+ genes per experiment with subcellular resolution through error-robust on platforms like MERSCOPE. This approach measures copy numbers in intact tissues, integrating sequential imaging and barcoding to profile large areas (up to 3 cm²) while maintaining compatibility with fixed samples like FFPE. Complementing this, NanoString's CosMx Spatial Molecular Imager supports whole-transcriptome analysis at single-cell and subcellular scales, with panels reaching 1,000-plex for targeted profiling of RNAs and proteins in the . These technologies overcome prior spatial limitations by providing multimodal data that links to tissue architecture, with demonstrated utility in immuno-oncology panels. Artificial intelligence and are transforming single-cell data analysis, with foundation models like scGPT pretrained on over 33 million cells to facilitate tasks such as denoising, annotation, and predictive modeling of cellular trajectories. scGPT's generative excels in perturbation response prediction and multi-omic integration, outperforming traditional methods in accuracy for downstream applications like network inference, though specific denoising benchmarks vary by dataset. approaches, including autoencoders for in scRNA-seq, achieve high fidelity in recovering true expression profiles, supporting that models dynamic processes like cell fate decisions. In vivo labeling techniques using barcoding have advanced tracing in animal models, introducing unique genetic mutations to track clonal histories alongside . Systems like dual-nuclease -Cas9/Cas12a enable high-resolution recording of cell divisions and states , applied in mammals such as pigs and mice to reveal developmental hierarchies and tumor evolution. These methods surpass earlier retrospective tracing by integrating barcodes with scRNA-seq, providing simultaneous and molecular snapshots in complex tissues. Looking ahead, single-cell sequencing is poised for routine clinical integration by 2030, driven by market expansion to $3.46 billion and increasing adoption in and for personalized diagnostics. Emerging wearables, such as CircTrek, enable real-time monitoring of circulating s at single-cell resolution, hinting at future synergies with portable sequencing for dynamic health tracking. Additionally, initiatives like the Cell Atlas aim to create global single-cell maps for non-model organisms, standardizing methodologies to benchmark cellular diversity across species and inform .

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