Epigenetic factors alter phenotype without changing genotype. A primary molecular mechanism underlying epigenetics is the alteration of chromatin structure by covalent DNA modifications, covalent histone modifications, and nucleosome reorganization. Remodeling of chromatin structure regulates DNA methylation, replication, recombination, and repair as well as gene expression. As these functions would predict, dysfunction of the proteins that remodel chromatin causes an array of multi-system disorders and neoplasias. Insights from these diseases suggest that during embryonic and fetal life, environmental distortions of chromatin remodeling encode a 'molecular memory' that predispose the individual to diseases in adulthood.