DNA (cytosine-5)-methyltransferases (DNMTs), such as DNMT1, maintain patterns of methylated cytosine residues in the mammalian genome. Genomic methylation patterns are reshaped during gametogenesis and early development and undergo programmed alterations during cellular differentiation. Methylation patterns are responsible for the repression of parasitic sequence elements and the expression status of genes subject to genomic imprinting and X inactivation. Faithful maintenance of methylation patterns is required for normal mammalian development, and aberrant methylation patterns are associated with certain human tumors and developmental abnormalities.A DNMT1 enzyme is thought to be responsible for most of the maintenance as well as the de novo methylation activities occurring in the somatic cells of vertebrates.