After the introduction in around 1980 of aminoglutethimide as the first aromatase inhibitor with documented anti-cancer efficacy (Samojlik et al., 1980), the third-generation aromatase inhibitors letrozole and anastrozole, and exemestane, are currently used for treatment of postmenopausal breast cancer (Smith and Dowsett, 2003). The gene discussed is CYP19A1; the disease is cancer.