PARP1 and cancer: While SLFN11 overexpression is used as a biomarker of PARP inhibitor sensitivity (Allison Stewart et al. 2017; Lok et al. 2017), SLFN11 is inactivated in 50% of cancers and frequently silenced due to promoter methylation (Nogales et al. 2016; Murai et al. 2018).