Of 295 cases with ER-positive CBC, 41 women (14%) carried at least 1 deleterious ATM, BRCA1, or BRCA2 variant, or CHEK2 1100delC, with approximately half of the 41 women carrying a deleterious variant in either BRCA1 or BRCA2. Of 580 matched controls to ER-positive CBC cases, only 35 women (6%) carried at least 1 deleterious ATM, BRCA1, or BRCA2 variant or CHEK2 1100delC. The gene discussed is ATM; the disease is complete blood cell count.