ESR1 and breast cancer: The hyper-methylation status of ESR1 has been reported previously in lung adenocarcinoma, breast cancer, prostate cancer, squamous cell cervical cancer, and CRC (Li et al., 2004; Lin et al., 2009; Kim et al., 2010; Elliott et al., 2013; Kirn et al., 2014; Martinez-Galan et al., 2014).