epigenome

If the genome is the full text of a book, the epigenome is the complete layer of highlighting, margin notes, and folded corners across every page. It is the entire set of epigenetic marks in a cell at a given time, telling that cell which parts of the shared text to read.

Concretely, the epigenome includes all the DNA methylation patterns, all the histone modifications, and the overall chromatin organization across the genome. Unlike the genome, which is essentially the same in every cell of a body, the epigenome differs from cell type to cell type and can change over a lifetime.

Mapping epigenomes is a major scientific effort, because comparing the epigenomes of healthy and diseased tissues, or of different developmental stages, reveals which regulatory states accompany which biological outcomes. Such maps are descriptive snapshots, not by themselves proof of cause.

Because it is shaped by both inherited programs and environmental signals, the epigenome is sometimes described as a bridge between the fixed genome and the changing world the organism lives in.