Local E2 levels are increased in endometriosis due to upregulation of the aromatase gene CYP19A1 [34] and reduction of 17-hydroxysteroid dehydrogenase type 2 (17HSD2), which normally (induced by P4) converts E2 to the less potent estrone [35, 36]. Here, CYP19A1 is linked to endometriosis.