ESR1 and cancer: Of these, ERα is the predominant subtype expressed in breast tumor tissue as it primarily stimulates cancer cell growth.25 Both receptors bind estrogen (17β-estradiol, E2) as ligand and patients with ER-positive tumors are currently treated with anti-hormone therapy utilizing either anti-estrogens (AE) interfering with ER signaling directly or blocking E2 synthesis with aromatase inhibitors.