When DNA damage is detected, checkpoint signal networks are activated to stop the cell cycle, and DNA repair processes begin. Inhibitory compounds targeting components of DNA damage response pathways have been identified and are being used in clinical trials, in combination with chemotherapeutic agents, to enhance cancer therapy. Inhibitors of checkpoint kinases, Chk1 and Chk2, have been shown to sensitize tumor cells to DNA damaging agents, and treatment of BRCA1/2-deficient tumor cells, as well as triple negative breast cancers, with poly(ADP-ribose) polymerase (PARP) inhibitors has shown promise. But systematic studies to determine which tumor subtypes are likely to respond to these specific inhibitors have not been reported. The current study was designed to test sensitivity of specific breast cancer subtype-derived cells to two classes of these new inhibitory drugs, PARP and Chk1 inhibitors. Luminal, HER2 overexpressing, and triple negative breast cancer-derived cells were tested for sensitivity to killing by PARP inhibitors, ABT-888 and BSI-201, and Chk1 inhibitor, PF-00477736, alone or in combination with gemcitabine or carboplatin. Each of the triple negative breast cancer cell lines showed strong sensitivity to the Chk1 inhibitor, but only the BRCA1-deficient breast cancer cell lines showed sensitivity to the PARP inhibitors, suggesting that in vitro testing of cancer cell lines of specific subtypes, with panels of the different PARP and Chk1 inhibitors, will contribute to stratification of patients for clinical trials using these classes of inhibitors.