CYP19A1 and endometriosis: Local E2 levels are increased in endometriosis due to upregulation of E2-producing p450 aromatase expression [170] and reduction of 17β-hydroxysteroid dehydrogenase type 2 (17βHSD2), which is normally induced by P4 to convert E2 to the less potent estrone but is decreased in P4-resistant conditions [171].