Aromatase inhibitors (AIs) are more effective than the antiestrogen tamoxifen at inhibiting the growth and proliferation of estrogen receptor (ER)-positive breast cancer [1] and these agents are now front-line treatments for postmenopausal women with hormone receptor-positive breast cancer in both the adjuvant and metastatic setting [2,3]. The gene discussed is CYP19A1; the disease is breast cancer.