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Charting Open Chromatin Landscapes with ATAC-Seq (Epigenetics Podcast Insights - Part 3)

By Stefan Dillinger, Ph.D.

June 5, 2026

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Introduction

Chromatin accessibility defines the regions of the genome that are open to regulatory factors and that drive patterns of gene expression in health and in disease. Over the past decade a simple but powerful method known as ATAC-Seq has transformed the way researchers probe the openness of chromatin. In this article we draw on insights from the Epigenetics Podcast to tell the story of how ATAC-Seq emerged as a fast accessible approach to map open chromatin, how it reveals nucleosome landscapes and how it has been extended to single‐cell resolution and integrated with other assays to chart regulatory mechanisms across diverse systems. Along the way we listen to the voices of those who pioneered the method and those who have brought it into service for labs around the world.

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Origins of ATAC-Seq

The genesis of ATAC-Seq sprang from a chance conversation in Will Greenleaf’s laboratory at Stanford University. As Jason Buenrostro recalls fresh into graduate school he and Will simply mixed hyperactive Tn5 transposase with unwashed cell pellets and ran the reaction out of sheer curiosity. To their astonishment the resulting DNA fragments displayed a ladder pattern corresponding to mono-, di, and tri-nucleosomes. That moment on a simple gel confirmed that Tn5 could both cut and tag accessible regions of chromatin in one step. After optimizing enzyme concentration, timing and temperature they published in 2013 the landmark paper naming the method assay for transposase accessible chromatin with high‐throughput sequencing or ATAC-Seq. Its elegance lays in compressing days of cross linking sonication and ligation into a single 35-minute reaction.

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Core Principles of Chromatin Accessibility

At the heart of ATAC-Seq is the hyperactive Tn5 transposase which carries preloaded adapters. When Tn5 encounters regions of the genome where nucleosomes are absent or chromatin is loosely packed it inserts adapters and cleaves the backbone simultaneously. Regions tightly wrapped around nucleosomes or bound by protein complexes block Tn5 access and remain untagged. Sequencing the adapter-tagged fragments yields a genome-wide map of accessible sites which often correspond to promoters enhancers and other regulatory elements.

The size distribution of fragments further encodes nucleosome positioning. Fragments under one hundred base pairs arise from nucleosome‐free regions while fragments of approximately one hundred fifty base pairs reflect DNA wrapped around single nucleosomes and larger fragments two or three times that size correspond to di- and tri-nucleosomes. This ladder allows fine resolution of chromatin architecture and reveals shifts during cellular processes.

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Bulk ATAC-Seq Workflow and Advantages

A typical bulk ATAC-Seq workflow begins with isolation of intact nuclei from fresh or frozen cells or tissues. As Yuan from Active Motif describes the service protocol: nuclear preparation is followed by a single transposition reaction using Tn5 adapters incubation for thirty-five minutes and direct progression to library amplification and sequencing. From start to finish the experiment can be completed in four to six hours using as few as fifty thousand cells which represents a thousand-fold reduction in input compared to DNase-Seq that required tens of millions of cells.

Simplicity eliminates multiple cleanup steps and reduces sample loss. Researchers save both time and material making accessibility profiling feasible for rare populations patient biopsies organoids or plant nuclei when a solid nuclear prep is possible. ATAC-Seq thus democratizes chromatin accessibility analysis for any lab with modest resources.

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Fragment Size Signatures and Quality Control

One of the earliest validations of ATAC-Seq involved observing clear nucleosomal ladders on a gel or bioanalyzer trace. Under-tagmentation yields few fragments and a flat profile while over-tagmentation produces excessive background from broken chromatin. Optimal tagmentation generates a prominent peak of sub-nucleosomal fragments around fifty base pairs followed by dampened peaks at mono-, di-, and tri-nucleosome sizes. This ladder serves as a rapid quality check ensuring the reaction hit its sweet spot.

Most workflows include a qPCR step to determine minimal PCR cycles. Typical protocols employ ten to eleven cycles for fifty thousand cells and up to thirteen cycles for lower inputs. With these precautions many labs reliably generate high-quality libraries that reflect genuine chromatin openness.

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Sequence Bias and Controls

Like all enzymatic assays Tn5 exhibits sequence preferences that can influence insertion frequency at certain motifs. Jason Buenrostro and colleagues characterized this bias early on and showed that while sequence preference exists in vitro the dominant signal in ATAC-Seq data still corresponds to regions of genuine accessibility. Proper design includes biological replicates input controls and differential analyses to account for technical variability. As a result most users need only ensure consistent sample handling and computational correction.

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Single-Cell ATAC-Seq

Bulk ATAC-Seq provides average profiles across tens of thousands of cells but masks heterogeneity crucial in development disease and complex tissues. To overcome this, early pioneers sorted individual nuclei after tagmentation and amplified each one by PCR. Proof-of-principle showed reproducible accessibility per nucleus and clustering recapitulated known cell types.

Commercial microfluidic platforms such as 10x Genomics then enabled barcoding of nuclei in droplets so each nucleus acquires a unique barcode during tagmentation. Pooled amplification and sequencing of thousands of cells in one run became feasible. Single-cell ATAC-Seq reveals continuous trajectories of chromatin opening and closing as cells differentiate and allows inference of lineage and transcription factor dynamics at single-cell resolution.

Active Motif service scientists note that with optimized protocols fewer than ten thousand nuclei suffice to cluster immune subsets from human blood in under two days of bench work. For clinical studies of tumor-infiltrating lymphocytes or neural populations single-cell ATAC-Seq offers unprecedented insight into cell-type-specific regulatory landscapes.

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Integrative Multi-omic Analyses

ATAC-Seq often serves as one piece of a multi-omic puzzle. Paired with RNA Seq accessibility data predict genes poised for transcription and complement expression profiles reflecting past regulatory events. Time-course designs capture dynamics in which opening of enhancers precedes bursts of expression. Coupling ATAC-Seq with CUT&RUN overlays maps of factor or histone modification binding atop accessible chromatin.

Pooled CRISPR screens combined with single-cell ATAC-Seq now enable functional dissection of regulatory elements at scale. As Viviana Risca described, by perturbing thousands of candidate enhancers in parallel and measuring chromatin accessibility changes across individual cells, researchers uncover distal circuits that drive cell-fate decisions.

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Looping Through Nuclear Architecture

While ATAC-Seq provided a window into chromatin accessibility Jane Skok emphasized the importance of three dimensional organization in gene regulation. In her episode she described how chromatin loops bring enhancers into physical proximity with their target promoters. These loops are anchored by architectural proteins such as CTCF and the cohesin complex and establish topologically associating domains or TADs that insulate regulatory interactions. Disruption of these domains can lead to misregulation of gene expression in development and disease.

Skok recounted experiments in which targeted degradation of cohesin subunits led to loss of loop structures and aberrant activation of proto oncogenes in mouse lymphocytes. These studies highlighted the dynamic nature of chromatin loops and underscored the need to study nuclear architecture in living cells. In parallel she discussed the emergence of CUT&RUN assays as a way to map protein specific footprints genome wide with minimal background compared to chromatin immunoprecipitation.

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Mapping Genome Folding with High Resolution

Marieke Oudelaar continued the discussion of nuclear organization by focusing on high resolution chromatin conformation capture approaches. She described how improvements in restriction enzyme based Hi-C and the advent of capture based methods allowed the interrogation of specific genomic regions at unprecedented depth. By designing oligonucleotide probes for promoters or enhancer clusters researchers enriched for interactions involving key regulatory elements and reduced sequencing cost.

Jane Skok emphasized the role of three-dimensional folding in gene regulation. She described how capture Hi-C with oligonucleotide probes targeting the immunoglobulin heavy-chain V, D and J segments in developing B-cells revealed stage-specific loops that correlate with V(D)J recombination and histone-modification changes. By combining those high-resolution interaction maps with ATAC-Seq data her lab showed that chromatin at enhancer elements becomes accessible before loops form between enhancers and promoters, providing a temporal model for how accessibility primes regulatory contacts in differentiating cells

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Integrating Methods to Build Comprehensive Maps

Mitch Guttman described the use of Capture Hi-C and single-cell ATAC-Seq to map interactions and accessibility in neural progenitor cells derived from human stem cells. By correlating loop strength with accessibility at both anchors and with expression of connected genes the study identified a set of enhancers that switch between active and poised states during differentiation. These elements often overlapped with binding sites for key transcription factors such as SOX2, POU5F1 and NEUROG2.

Jane Skok emphasized the importance of time course designs for capturing dynamic changes in nuclear architecture and accessibility. By sampling cells at multiple time points after differentiation cues researchers observed waves of loops forming and dissolving in synchrony with bursts of gene expression. Such designs require methods that work at high throughput and with low input and here single-cell ATAC-Seq and capture based conformation assays shine.

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Challenges and Future Directions

Despite its power ATAC-Seq faces challenges that our podcast guests have highlighted. As Yuan from Active Motif explained single-cell assays trade sequencing depth per cell for the ability to profile thousands of nuclei often undersampling rare populations.

Researchers like Viviana Risca have pursued improvements in barcoding chemistry and Tn5 engineering. She mentions the development of an open-source protocol for a multi-purpose hyperactive Tn5, which enables labs to customise transposase formulations. Commercial microfluidic platforms such as 10x Genomics chips streamline single-cell workflows eliminating specialized equipment beyond standard lab instruments.

Jason Buenrostro highlighted the need for advanced computational tools capable of integrating ATAC-Seq with other modalities calling for algorithms to call nucleosome positions accurately and detect transcription factor footprints. Enzyme sequence bias remains a concern as Buenrostro’s team showed robust controls and normalization are essential to distinguish true accessibility from artifacts. Mitch Guttman’s split-pool recognition strategy offers one orthogonal approach to map three-dimensional contacts without heavy reliance on restriction enzymes.

While we did not discuss combining ATAC Seq with DNA methylation profiling the community recognizes that orthogonal assays such as Hi-C or methylome sequencing complement accessibility data for richer models of genome regulation. Emerging multi-omic platforms promise joint measurement of four or more modalities and ongoing algorithmic advances aim to extract footprints and nucleosome phasing at base-pair resolution pushing the boundaries of chromatin dynamics analysis

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Conclusion

From a simple naive experiment in a Stanford lab to global service offerings ATAC-Seq has revolutionized chromatin accessibility profiling. By harnessing Tn5 to cut and tag accessible DNA in one step ATAC-Seq compresses days of work into hours and reduces input requirements by orders of magnitude. Fragment size signatures reveal nucleosome landscapes and extensions to single-cell resolution uncover regulatory heterogeneity in complex tissues. Integration with multi-omic assays maps transcriptional potential circuits and disease biomarkers in unprecedented detail.

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References

1. Dillinger, S. (2021, July 22). ATAC-Seq, scATAC-Seq and Chromatin Dynamics in Single-Cells (Jason Buenrostro). Epigenetics Podcast. https://www.activemotif.com/podcasts-jason-buenrostro
2. Dillinger, S. (2021, November 11). Chromatin Organization During Development and Disease (Marieke Oudelaar). Epigenetics Podcast. https://www.activemotif.com/podcasts-marieke-oudelaar
3. Dillinger, S. (2021, November 18). Spatio-Temporal Alterations in Chromosome Dynamics (Jane Skok). Epigenetics Podcast. https://www.activemotif.com/podcasts-jane-skok
4. Dillinger, S. (2022, July 14). Multiple challenges of ATAC-Seq, Points to Consider (Yuan Xue). Epigenetics Podcast. https://www.activemotif.com/podcasts-yuan-xue
5. Dillinger, S. (2024, February 22). Split-Pool Recognition of Interactions by Tag Extension (SPRITE) (Mitch Guttman). Epigenetics Podcast. https://www.activemotif.com/podcasts-mitch-guttman
6. Dillinger, S. (2025, March 13). Using RICC-Seq to Probe Short Range Chromatin Folding (Viviana Risca). Epigenetics Podcast. https://www.activemotif.com/podcasts-viviana-risca
7. Buenrostro, J. D., Giresi, P. G., Zaba, L. C., Chang, H. Y., & Greenleaf, W. J. (2013). Transposition of native chromatin for fast and sensitive epigenomic profiling of open chromatin, DNA-binding proteins and nucleosome position. Nature Methods, 10(12), 1213–1218. https://doi.org/10.1038/nmeth.2688
8. Oudelaar, A. M., Davies, J. O. J., Downes, D. J., Higgs, D. R., & Hughes, J. R. (2017). Robust detection of chromosomal interactions from small numbers of cells using low-input Capture-C. Nucleic Acids Research, 45(22), e184–e184. https://doi.org/10.1093/nar/gkx1194
9. Lhoumaud, P., Sethia, G., Izzo, F., Sakellaropoulos, T., Snetkova, V., Vidal, S., Badri, S., Cornwell, M., Di Giammartino, D. C., Kim, K.-T., Apostolou, E., Stadtfeld, M., Landau, D. A., & Skok, J. (2019). EpiMethylTag: Simultaneous detection of ATAC-seq or ChIP-seq signals with DNA methylation. Genome Biology, 20(1), 248. https://doi.org/10.1186/s13059-019-1853-6
10. Perez, A. A., Goronzy, I. N., Blanco, M. R., Yeh, B. T., Guo, J. K., Lopes, C. S., Ettlin, O., Burr, A., & Guttman, M. (2024). ChIP-DIP maps binding of hundreds of proteins to DNA simultaneously and identifies diverse gene regulatory elements. Nature Genetics, 56(12), 2827–2841. https://doi.org/10.1038/s41588-024-02000-5
11. Goronzy, I. N., Quinodoz, S. A., Jachowicz, J. W., Ollikainen, N., Bhat, P., & Guttman, M. (2022). Simultaneous mapping of 3D structure and nascent RNAs argues against nuclear compartments that preclude transcription. Cell Reports, 41(9), 111730. https://doi.org/10.1016/j.celrep.2022.111730
12. Risca, V. I., Denny, S. K., Straight, A. F., & Greenleaf, W. J. (2017). Variable chromatin structure revealed by in situ spatially correlated DNA cleavage mapping. Nature, 541(7636), 237–241. https://doi.org/10.1038/nature20781

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About the author

Stefan Dillinger, Ph.D.

Stefan was born in the Free State of Bavaria, Germany. After studying biochemistry in Ulm and Regensburg, he got his Ph.D. in the field of epigenetics, studying the distribution of heterochromatin around nucleoli during cellular senescence. As a graduate student he started his own German science podcast “The Random Scientist” and is now the host of Active Motif’s Epigenetics Podcast. When Stefan is not working at Active Motif or recording podcasts, he is a passionate runner (he finished the New York City Marathon in 3 hours 21 minutes!!) and loves to spend time with his wife and son.

Contact Stefan on LinkedIn with any questions, or to get running advice.

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