ESR1 and breast cancer: Based on previous reports demonstrating elevated oxidative stress in ERα-positive, tamoxifen resistant breast cancer [5,8,27] and the role of PARP1 in responding to oxidative damage [28,29], we hypothesized that combining the antiestrogen tamoxifen with a PARPi would sensitize tamoxifen resistant, ERα-positive breast cancer to tamoxifen therapy.