ESR1 and neoplasm: Approximately 70% of BCs express ERα66, which promotes resistance to antiestrogen treatments.46 The expression levels of ERα36 in ER-positive and ER-negative tumors are associated with histological grade and metastasis.47 Research has shown that in the TNBC subtype, the expression level of ERα36 is correlated with tumor growth, progression and metastasis.46