Recently, melatonin has been shown to regulate the activity of some enzymes (aromatase, sulfatase, 17ß-hydroxysteroid dehydrogenase, and estrogen sulfotransferase) responsible for the local synthesis of estrogens in cultured human breast cancer cells, thus behaving as a selective estrogen enzyme modulator (SEEM) [55–57]. Here, SULT1E1 is linked to breast cancer.