Genetic predisposition to breast cancer may range from significant familial clustering of breast cancer due to underlying moderate (CHEK2, BRIPI, ATM, PALB2) or high (BRCA1, BRCA2, PTEN, TP53, CDH1, STK11) penetrance genetic mutations to multiple low penetrant alleles that contribute to risk in a polygenic fashion due to multiple single nucleotide polymorphisms. The gene discussed is ATM; the disease is breast cancer.