To determine the methylation status of the p16+/−;Brca1MGKO mammary tumors, we performed methylation-specific PCR (MS-PCR) and detected methylation of the p16 promoter in three tumors with reduced expression of p16, but not in tumor 1255 in which p16 expression was not decreased (Figure 4C). Here, CDKN2A is linked to breast cancer.