Methylated silencing of proto-oncogenes or other cancer promoting genes (e.g., H-RAS) could be reversible and lead to the activation of such genes.[76] Traditionally DNA MTases are divided into two categories, de novo enzymes which are responsible for establishing new methylation patterns and maintenance enzymes which essentially replicate the existing patterns.[77] DNA MTase 1 (DNMT1) is a maintenance enzyme, and DNA MTase 3a and 3b (DNMT3a and DNMT3b) perform de novo functions. The gene discussed is DNMT1; the disease is cancer.