The FDA approved three small-molecule CDK4/6 inhibitors—palbociclib (Ibrance, Pfizer, 2015), ribociclib (Kisqali, Novartis, 2017) and abemaciclib (Verzenio, Eli Lilly, 2017)—together with aromatase inhibitors (i.e., letrozole) to treat specific breast cancer types (Table 4). The gene discussed is CYP19A1; the disease is breast cancer.