DNA (cytosine-5)-methyltransferase 1 is an enzyme that catalyzes the transfer of methyl groups to specific CpG structures in DNA, a process called DNA methylation. This enzyme is responsible for maintaining DNA methylation, which ensures the fidelity of this epigenetic patterns across cell divisions. In line with this role, it has a strong preference towards methylating CpGs on hemimethylated DNA. However, DNMT1 can catalyze de novo DNA methylation in specific genomic contexts, including transposable elements and paternal imprint control regions. Aberrant methylation patterns are associated with certain human tumors and developmental abnormalities.