PARP1 and cancer: The rationale for using PARP inhibitors (PARPi) to treat HR-deficient cancers is based on the exquisite sensitivity of BRCA1- or BRCA2-defective cells to small-molecule PARPi, as well as the ability of Parp1 gene silencing to selectively inhibit Brca1- or Brca2-defective cells1, 2.