Exemestane is an irreversible steroidal inhibitor of aromatase [30,31] that was recently shown to confer both an overall and a disease-free survival advantage when given after 2 to 3 years of tamoxifen compared with the standard 5 years of tamoxifen in the adjuvant treatment of postmenopausal breast cancer patients [8,32]. The gene discussed is CYP19A1; the disease is breast carcinoma.