DNMT3B and cancer: DNMT1 maintains the levels and patterns of methylated DNA during mitosis, whereas DNMT3A and DNMT3B are primarily responsible for de novo methylation.3,11,12 De novo hypermethylation of promoter CpG islands has been identified as a possible mechanism for tumor suppressor gene inactivation in human cancer cells.13,14 DNMT3B plays an important role in tumorigenesis, and overexpression of DNMT3B has been reported in tumors.