DNA methyltransferase

DNA methyltransferase (DNA MTase, DNMT) is a "family" of enzymes, They catalyze the transfer of a methyl group to DNA. 
DNA methylation has many biological functions.
Mammals.
Three active DNA methyltransferases have been identified in mammals. They are named DNMT1, DNMT3a, and DNMT3b. Recently, a fourth enzyme DNMT3c has been discovered specifically expressed in the male germline in the mouse. 
What these enzymes do is make DNA methylation work. DNA methylation is how methyl groups are added to the DNA molecule. "Methylation can change the activity of a DNA segment without changing the sequence". When positioned in a gene promoter, DNA methylation acts to stop gene transcription.