PARP1 and breast cancer: Reduced RAD51 foci formation indicates HR deficiency and correlates with PARP inhibitor sensitivity, as shown in BRCA mutated breast tumor samples 2 h after 5 Gy of IR (Naipal et al. 2014), ovarian cancer cell lines 8 h after 4 Gy of IR (Shah et al. 2014), and breast cancer patient-derived xenografts (PDXs) (Castroviejo-Bermejo et al. 2018; Cruz et al. 2018).