ESR1 and breast cancer: However, the overall effectiveness of single agent PARP inhibitors in other cancer types has been relatively disappointing (Lord & Ashworth, 2012), although larger studies are required to conclusively assess the performance of PARP inhibitors in diseases such as triple-negative (TN) breast cancer, where tumours are characterized by an absence of estrogen receptor (ER) and progesterone receptor (PR) expression as well as an absence of HER2 gene amplification (Foulkes et al, 2010).