We show that 11β-hydroxy-17β-estradiol binds and activates estrogen receptors α and β and that 11β-hydroxy-17β-estradiol and the classic androgen pathway-derived active estrogen, 17β-estradiol, are equipotent in stimulating breast cancer cell line proliferation and expression of estrogen-responsive genes. Here, ESR1 is linked to breast carcinoma.