A frequent mutation in hormone receptor–positive (HR+)/human epidermal growth factor receptor 2-negative (HER2−) breast cancer is alteration in the PIK3CA gene (encoding the phosphatidylinositol 3-kinase [PI3K] α isoform), which is mutated in approximately 40% of patients [1–4]. The gene discussed is PIK3CA; the disease is breast carcinoma.