ESR1 and cancer: Estrogen is regarded as a major determinant of BCa and can participate in the whole process of BCa development through both estrogen receptor (ER)-dependent and -independent pathways (Sommer and Fuqua, 2001; McDonnell and Norris, 2002; Ye et al., 2008; Saha Roy and Vadlamudi, 2012), including increasing cancer cell proliferation (Tian et al., 2018; Kumar et al., 2019), stimulating angiogenesis (Gupta et al., 2007), promoting metastasis (Jiang et al., 2018), and inducing chemoresistance (Zatelli et al., 2009; Zeng et al., 2016).