Aromatase inhibitors (AIs) have proved to be very successful clinically in the treatment and prophylaxis of hormone dependent breast cancer,1 which accounts for approximately 70% of all breast cancers.2 Although very effective, resistance to AIs in addition to the side effects associated with both hormone ablation and off target effects3,4 supports the further development of AIs that are as effective as the currently used clinical AIs but with improved selectivity to reduce unwanted side effects. The gene discussed is CYP19A1; the disease is breast cancer.