Aromatase inhibitor use reduced the risk of breast cancer by 53% (RR 0.47; 95% CI 0.35–0.63) but increased the risk of severe toxicity by 18% (RR 1.18; 95% CI 1.09–1.28), especially hot flashes, diarrhea, and arthralgia [15]. The gene discussed is CYP19A1; the disease is breast cancer.