RAD51 and cancer: As reported in the paper entitled “RAD51 mediates resistance of cancer stem cells to PARP inhibition in triple-negative breast cancer” [7] in a recent issue of Clinical Cancer Research, we found that PARP inhibitor (PARPi) only effectively targets BRCA1-mutant bulk tumor cells and RAD51, a gene involved in DNA double-strand break repair [8], mediates this process, but BRCA1-wild-type cancer cells and BRCA1-mutant CSCs are resistant to PARP inhibition.