ATM and breast cancer: Moreover, Studies have shown that hypermethylation of promoters in tumor suppressor genes, such as CCDN2 (Cyclin D2), P16 (cyclin-dependent kinase inhibitor 2A), ATM (ataxia telangiectasia mutated), RASSF1A (Ras Association Domain Family Member 1A), APC (Adenomatous Polyposis Coli), and BRCA1 (breast cancer gene 1), is commonly observed in breast cancers.