The two cases with breast tumors also each carried an additional deleterious germline mutation: one in TP53 and the other in CHEK2. Mutational signature analysis of the 76 tumors indicated that spontaneous deamination of 5-methylcytosine and activity of the APOBEC cytidine deaminase protein family are major causes of mutagenesis. This evidence concerns the gene CHEK2 and breast neoplasm.