ATAC-Seq (Assay for Transposase-Accessible Chromatin using sequencing) 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 gene regulation and cellular function by mapping the locations of open chromatin across different cell types or conditions. ATAC-Seq is valuable in fields like genomics and epigenetics, helping scientists understand how genes are turned on or off.