question: Which are the families of mammalian DNA-(cytosine-5)-methyltransferases?
answer: DNA (cytosine-5)-methyltransferases catalyze the specific transfer of a methyl group  to the C5 position of cytosine residues in DNA. Three families of DNA (cytosine-5)-methyltransferases have been identified in mammals: DNMT1, DNMT2 and DNMT3 (including DNMT3a, DNMT3b and DNMT3L isoforms). All of them share homologous catalytic domains. DNMT1 is the ���maintenance” methyltransferase family. DNMT1 is specific for hemi-methylated DNA and ensures the faithful transmission of DNA methylation patterns in every replication cycle. DNMT3 is required for de novo methylation of DNA. DNMT3 targets unmethylated DNA and is responsible for the establishment of new methylation patterns. Dnmt2, in contrast to all other mammalian DNA (cytosine-5)-methyltransferases, does not possess a large N-terminal regulatory domain. The DNA methylation activity of DNMT2 is still controversial.