The DNA damage response kinase ATR and its effector kinase CHEK1 are required for cancer cells to survive oncogene-induced replication stress. ATR inhibitors exhibit synthetic lethal interactions, with deficiencies in the DNA damage response enzymes ATM and XRCC1 and with overexpression of the cell cycle kinase cyclin E. Here, we report a systematic screen to identify synthetic lethal interactions with ATR pathway–targeted drugs, rationalized by their predicted therapeutic utility in the oncology clinic. We found that reduced function in the ATR pathway itself provided the strongest synthetic lethal interaction. In addition, we found that loss of the structure-specific endonuclease ERCC1-XPF (ERCC4) is synthetic lethal with ATR pathway inhibitors. ERCC1-deficient cells exhibited elevated levels of DNA damage, which was increased further by ATR inhibition. When treated with ATR or CHEK1 inhibitors, ERCC1-deficient cells were arrested in S-phase and failed to complete cell-cycle transit even after drug removal. Notably, triple-negative breast cancer cells and non–small cell lung cancer cells depleted of ERCC1 exhibited increased sensitivity to ATR pathway–targeted drugs. Overall, we concluded that ATR pathway–targeted drugs may offer particular utility in cancers with reduced ATR pathway function or reduced levels of ERCC4 activity. Cancer Res; 74(10); 2835–45. ©2014 AACR.