Dnmt3b methylates the 5 position of cytosine in DNA. Dnmt3b catalyzes de novo methylation of cytosine residues (Okano et al. 1998, Hsieh 1999) in CG sequences (Hsieh 1999) and in non-CG sequences (Arand et al. 2012). Methylation of C residues in non-CG contexts is observed only in embryonic stem cells and appears to be catalyzed by Dnmt3b and Dnmt3a acting with Dnmt3l (Arand et al. 2012).