CDKN2A and skin papilloma: These results, together with a recent observation that DNMT1 possesses an activity to enhance H3K9 methylation through interacting with G9a, a major H3K9 mono- and di- methyltransferase [48], suggest that DNMT1 serves to counterbalance the activation of the p16INK4a gene promoter mediated by oncogenic Ras during skin papilloma development.