The Assay for Transposase-Accessible Chromatin using sequencing, commonly known as ATAC-seq, is a technique used to study the accessibility of chromatin in the genome. It identifies regions of DNA that are open and accessible to regulatory proteins, which can indicate active gene expression. By using a transposase enzyme, ATAC-seq inserts sequencing adapters into these accessible regions, allowing researchers to analyze the chromatin landscape. This method provides insights into the regulatory elements of the genome, such as promoters and enhancers, and helps in understanding how genes are regulated in different cell types or conditions. ATAC-seq is valuable in fields like developmental biology, cancer research, and epigenetics, as it reveals how chromatin structure influences gene activity.