Though the remaining WT allele of p16 was retained in all p16+/−; Brca1MGKO mammary tumors, the p16 promoter was methylated and p16 mRNA was significantly reduced in most of these tumors, indicating that silencing of p16 by promoter methylation plays a role in the development of, at least, some of the p16+/−;Brca1MGKO mammary tumors. The gene discussed is CDKN2A; the disease is breast cancer.