The problem. Which parts of the genome are available to be read — nucleosome-free, bound by regulators, poised for transcription — is central to what a cell is doing. Earlier methods to map open chromatin (DNase-seq, FAIRE) needed large amounts of material and fussy protocols, putting accessibility profiling out of reach for small or precious samples.

The idea. ATAC-seq uses a hyperactive Tn5 transposase preloaded with sequencing adapters: it inserts itself preferentially into open chromatin and tags those regions in a single enzymatic step. From tens of thousands of cells — later, single cells — you get a genome-wide map of accessibility quickly and cheaply.

Why it matters. ATAC-seq is the accessibility counterpart to the expression assays I already run, and a pillar of the multi-omic single-cell world the reading list is heading toward. It measures the regulatory layer that ENCODE catalogued and that transcription factors act on — the “why” behind an expression change, not just the change. Understanding the assay is understanding what those peaks mean.

Verdict. Foundational and now everywhere, especially in single-cell (scATAC) and spatial variants. Read it for the elegance of the Tn5 trick — one enzyme replacing a whole protocol — and for how it slots accessibility into the omics stack.