DNA methyltransferase (DNMT) is an important family of enzymes that catalyze and maintain DNA methylation in epigenetics. The enzymes play a key role in the regulation of gene expression and genomic imprinting/development.

Three groups of DNMTs have been found in mammals:
1. DNMT1 - A key enzyme in DNA replication and the maintenance of DNA methylation.
2. DNMT2 - Mainly acts as a tRNA methyltransferase.
3. DNMT3 - Contains three subunits: 3A and 3B are de novo DNA methyltransferases, and 3L is a regulatory protein that plays a role in embryonic development, gene imprinting, and chomosome inactivation.

DNMT1, DNMT3A and DNMT3B form a protein complex that interacts with histone deacetylases (HDAC1, HDAC2, Sin3A), transcriptional repressors (RB, TAZ-1), and heterochromatins (HP1, SUV39H1) to promote gene silencing and maintain normal levels of DNA methylation. DNMT1 is one of the most important and currently one of the most-studied enzymes in the DNMT family. Numerous studies have found that DNMT1 is involved in the abnormal methylation of DNA, and the occurrence/development of tumors.