PARP1 and breast cancer: A striking observation in clinical trials has been that despite a similar spectrum of oncogenic mutations, the efficacy of PARP inhibition is different in tumors depending on the tissue of origin: in the first phase 2 study of olaparib, the rate of partial remissions was 46% and 24% in patients with ovarian cancer with and without a BRCA1/2 mutation, respectively, while in the same study, none of the patients with breast cancer (BC), including 10 BRCA1/2 mutation carriers, had any clinical benefit (Gelmon et al., 2011).