In Part 1 of this series, Dr. Howard Chang introduces epigenomics, the study of DNA regulatory mechanisms that determine which genes are turned on or off in cells at specific times. The epigenome integrates signals from the environment to modify expression of the DNA blueprint inherited from an individual’s parents. Chang’s lab has pioneered techniques to map the landscape of chromatin, the complex of DNA, RNA and protein that organizes the genome and regulates gene expression. One example is ATAC, the Assay of Transposase Accessible Chromatin, which uses a bacterial transposase to mark open chromatin and identify genes that are likely turned “on”.