PARP1 and cancer: PARP inhibitors (PARPis) were shown to be particularly toxic for cells deficient in homologous recombination (HR) repair factors BRCA1 (Breast cancer type 1 susceptibility protein) and BRCA2 (Breast cancer type 2 susceptibility protein) even in the absence of exogenous DNA damage, a phenomenon with great therapeutic potential because of the high prevalence of BRCA deficiency in tumor cells (6–8).