As we have seen, epigenetic information is contained within chromatin structure, and that information is heritable and affects how the DNA base sequence is expressed. The overall pattern of chromatin modifications in a genome has been termed the epigenome.

Over the past few years, a number of techniques have become available for detecting and describing epigenetic modifications across the genome. Using these techniques, geneticists have compared the epigenomes of different types of cells. For example, researchers have compared the epigenomes of cancer cells and normal cells and have observed distinct epigenetic marks associated with cancer. Similarly, researchers have mapped the genomic locations of histone modifications in different cell types. These studies have detected specific histone modifications associated with promoters and enhancers of active genes. In one study, researchers mapped nine different epigenetic marks in nine different types of human cells. They were able to determine how the chromatin marks varied across cell types and to compare the epigenetic marks associated with active and repressed genes. Because specific epigenetic marks are often associated with regulatory elements such as promoters and enhancers, researchers have used the presence of these marks to map the locations of these regulatory elements throughout the genome.