Early work established that PARPi resistance can develop in a subset of breast cancer patients by reversion mutations in the BRCA1/2 genes that restore HR function and thereby abrogate cellular dependence on PARP1/2 (Barber et al, 2013; Lin et al, 2019; Pettitt et al, 2020; Tobalina et al, 2021; Waks et al, 2020). Here, PARP1 is linked to breast cancer.