ESR1 and breast cancer: Antiestrogens with improved potencies including bazedoxifene, fulvestrant, OP-1074 and GDC-0945 possess variable degrees of ERα degrading/downregulating activities but show significant potencies and efficacies in mutant ESR1 breast cancer cells (Toy et al., 2013; Metcalfe et al., 2019; Fanning et al., 2018a).