PARP1 and neoplasm: In de novo PARP-inhibitor-resistant tumor cells, the restoration of HR is one mechanism, as has been demonstrated after secondary mutation of the BRCA protein in UWB1.289, CAPAN1, and SUM149 parental cells or the knockdown of 53BP1 in UWB1.289 and COV362 cells [105, 149, 150].