Apart from some molecular mechanisms (10), resistance is often due to low PARP enzyme expression or to the inability of the drug to penetrate tumor tissue or part of the tumor tissue, because of increased interstitial pressure and desmoplasia—especially relevant in pancreatic adenocarcinomas—or an intact blood–brain barrier, in the case of brain tumors or brain metastases. This evidence concerns the gene PARP1 and neoplasm.