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. This evidence concerns the gene ESR1 and cancer.