CYP19A1 and breast carcinoma: 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).