ATM and breast cancer: We previously demonstrated the formation of stalled TOP2ccs during the early response to E2 and androgen in the mammary and prostate epithelial cells, respectively, of TDP2-deficient mice.60,117 In this study, we revealed that the loss of ATM also increased E2-induced DSB formation in murine mammary epithelial cells (Figure 1) and human BC cells (Figure 2).