Postmenopausal women with a BMI greater than 30 kg/m2 and ERα+ breast cancers treated with aromatase inhibitors anastrozole or letrozole showed significantly decreased treatment efficacy compared to breast cancer patients with BMI in the healthy range [29, 42–44], yet no difference has been observed in the efficacy of the hormone therapy tamoxifen in breast cancer patients with differing BMI [43, 45]. The gene discussed is CYP19A1; the disease is breast cancer.