Numerous studies (Issa et al, 1993; Robertson et al, 1999) have evaluated the expression of DNMTs in tumour tissue, and most have reported variable levels of overexpression, particularly of DNMT1 and DNMT3b. Rhee et al (2002) demonstrated that DNMT1 and DNMT3b cooperatively maintain DNA methylation and gene silencing in human cancer cells. This evidence concerns the gene DNMT1 and cancer.