DNMT1 and DNMT3b are known to catalyse DNA methylation in cancer cells (Rhee et al, 2002; Toyota et al, 2003); the mechanism by which DNA methylation represses gene expression is not fully understood, however, although histone deacetylation reportedly plays a key role (Bird and Wolffe, 1999; Cameron et al, 1999; Nguyen et al, 2001). The gene discussed is DNMT3B; the disease is cancer.