DNA Methylation de Novo

One way in which the cell silences genes is to add methyl groups to cytosine (C) bases that are located next to guanine (G) bases in the DNA. The enzymes responsible for this are called the DNA methyltransferases (Dnmts). In a Perspective, Bird discusses several recent reports that identify two new Dnmts (Dnmt3a and 3b) and demonstrate that these enzymes are able to add methyl groups to DNA that has never been methylated. In contrast, the well-characterized Dnmt1 is now shown to be a maintenance methyltransferase that adds methyl groups to DNA that has not been methylated before.