4-hydroxy Androstenedione (4-HAD) is a steroidal inhibitor of aromatase (also known as cytochrome P450 19A1; Ki = 27 nM). As aromatase catalyzes the conversion of androgens to estrogens, aromatase inhibitors, including 4-HAD, are used against hormone-sensitive breast cancer in menopausal women. They are also abused in combination with anabolic steroids in racehorses and athletes. This product is intended for forensic and research applications.