Abstract Biosynthesis of all androgens and estrogens from cholesterol requires CYP11A1 and CYP17A1. There is no known pathway in humans or other vertebrates that circumvents CYP17A1 for androgen or estrogen biosynthesis in physiology or disease. However, CYP17A1 inhibition cannot completely inhibit androgen biosynthesis in prostate cancer. Here, we identify a surprising role for CYP51 in androgen biosynthesis that bypasses the requirement for CYP17A1.