PARP1 and cancer: MUS81 defects sensitize cells to various genotoxic chemicals [102,103], and it was recently shown that inhibition of MUS81 sensitizes HR-proficient cancer cells to the PARP1 inhibitor, olaparib [104], an agent commonly used to treat cancers with HR defects, such as BRCA1- and BRCA2-defective breast cancers [105,106].