Women carrying germline mutations of the ATM gene are predisposed to ER+ BCs when LOH inactivates the ATM gene.4–6,8–10 LOH events occur at extremely low frequency (approximately 10−5 per base) in cells of malignant tumors.125 However, once LOH occurs, the resulting ATM loss should dramatically drive selective oncogenesis in the estrogen-regulated tissues. Here, ATM is linked to cancer.