Similar to the wild-type and ATM−/− mouse mammary epithelial cells (Figure 1), only ATM−/−, but not wild-type BC cells showed prominent induction of DSBs at 2 h and virtually no resolution of 53BP1 foci, even at 22 h after the removal of E2 (p < 0.005) (Figures 2B, 2C, and S2C). This evidence concerns the gene TP53BP1 and breast cancer.