ATR and breast cancer: The treatment of breast cancer cell lines (MCF7 and SkBr3) and colon cancer cell line (HCT-116) with the IC50 and twice the IC50 of SIMR1281 for 24 h increased the level of γ-H2Ax and p-ATM, while a minimal effect was detected in the phosphorylation level of ATR (Figure 3a, Figure S3).