Epigenetics studies heritable changes in phenotype that do not involve changes to DNA sequence. Epigenetic modifications include chemical changes to DNA and histones that regulate gene expression. DNA methylation permanently silences genes by marking promoters, while histone modifications transiently activate or repress genes depending on the location and type of modification. During cell differentiation, epigenetic marks program cell identity by silencing irrelevant genes through DNA and histone methylation, while activating cell-specific genes using histone acetylation and phosphorylation. The heritability of these epigenetic marks allows cell types to maintain their gene expression programs over generations.