We assessed DNA methylation of the tumor and blood-derived samples from 43 women with early-onset breast cancer, across the genomic regions containing the breast and ovarian cancer susceptibility genes BRCA1, BRCA2, PALB2, TP53, ATM, CDH1, CHEK2, FANCM, MLH1, MSH2, MSH6, and PMS2. This evidence concerns the gene ATM and breast cancer.