CYP19A1 and endometriosis: Endometriosis typically involves higher levels of E2 than observed in a normal endometrium, which is due to higher gene expression levels of aromatase and 17β-hydroxysteroid dehydrogenase type 1 (91); the higher levels of E2 result in increased E2 binding and activation of ERs in endometriotic tissues, thereby stimulating estrogen-dependent growth.