ESR1 and breast neoplasm: However, despite the fact that current SOC endocrine therapies mainly target the ER, the ER remains a viable target after the onset of resistance to SOC endocrine therapy (Riggins et al., 2007; Rondón-Lagos et al., 2016; Luque-Bolivar et al., 2020; Yao et al., 2020), suggesting that the addition of another ER-targeted anti-cancer agent, such as SM6Met (Visser et al., 2013; Oyenihi et al., 2018), in combination with current SOC endocrine therapies, like tamoxifen, could prove effective for overcoming breast tumour resistance to tamoxifen.