4-Androsten-4-ol-3,17-dione, commonly known as 4-hydroxyandrostenedione or formestane, is a steroidal aromatase inhibitor used primarily in pharmaceutical research and the treatment of hormone-dependent conditions, such as estrogen-receptor positive breast cancer. It acts as a suicide substrate for the aromatase enzyme, irreversibly inhibiting the conversion of androgens to estrogens, thereby reducing estrogen levels in the body. This compound serves as a valuable tool in studying estrogen biosynthesis pathways, endocrine disorders, and the development of targeted therapies for cancers and other hormonal imbalances. Historically, it was developed and marketed (as Lentaron) for clinical use via intramuscular injection. Its applications are focused on laboratory, preclinical, and therapeutic settings to advance oncology and endocrinology.