Regulatory approval of alpelisib, and its inclusion in breast cancer treatment guidelines [1,2], was based on the phase 3 SOLAR-1 study of alpelisib (300 mg/day) plus fulvestrant versus placebo plus fulvestrant in patients with HR+, HER2– breast cancer that had progressed on or after aromatase inhibitor (AI) therapy [9]. This evidence concerns the gene CYP19A1 and breast cancer.