Letrozole, a non-steroidal aromatase inhibitor that blocks the conversion of androgens to estrogens resulting in increased endogenous testosterone level in the ovary (8), adrenal gland, and other peripheral tissues (12), and has proven to be a successful tool for making PCOS models in rodents (13–15). The gene discussed is CYP19A1; the disease is polycystic ovary syndrome.