ESR1 and breast cancer: Estrogen receptor alpha–positive (ERα+) subtype accounts for approximately 80% of all breast cancers, which is the most common cancer in women.21 Up to 50% of ERα+ primary BC lose ERα expression in recurrent tumours, conferring resistance to tamoxifen therapy.22 Inactivation of ESR1 gene via methylation strongly correlates with poor prognosis as well as an aggressive phenotype in TNBC.22 Additionally, ERα can be complexed with OCT4 to promote tamoxifen resistance in breast cancer cells.23