ESR1 and breast neoplasm: Investigations into the mechanism of tamoxifen resistance has identified the ER as a valuable therapeutic target for overcoming tamoxifen resistance, suggesting that the addition of another ER-targeted anti-cancer agent in a combined therapy could prove effective in overcoming breast tumour resistance to tamoxifen (Riggins et al., 2007; Rondón-Lagos et al., 2016; Yao et al., 2020).