Dnmt1(DNA (cytosine-5-)-methyltransferase 1 ) is one of the most abundant DNA methyltransferase in mammalian tissues, where it associates with the replication machinery and restores symmetrical methylation at hemimethylated CpG sites generated by the semi-conservative DNA replication process. Dnmt1 comprises a regulatory N-terminal region and a C-terminal catalytic domain connected by a linker of seven glycine-lysine repeats. The N-terminal part contains a PCNA binding domain (PBD), a heterochromatin targeting sequence (TS), a CXXC-type zinc finger domain and two Bromo-Adjacent Homology domains (BAH1 and BAH2). The C-terminal domains of mammalian Dnmts contain all ten catalytic motifs identified in bacterial DNA (cytosine-5) methyltransferases. Dnmt1 maintains methylation patterns with high fidelity and is essential for embryonic development and genome integrity. The molecules interacting with Dnmt1, including RNA polymerase II, some RNA-binding proteins, and some specific Dnmt1-inhibitory RNA molecules are involved in chromatin organization, DNA repair, cell cycle regulation, and apoptosis.