One Japanese Phase II study is evaluating the safety and efficacy of the phosphatidylinositol-4,5-bisphosphate 3-kinase catalytic subunit alpha (PIK3CA) inhibitor, alpelisib, in combination with fulvestrant in men and postmenopausal women with ER/PR-positive, HER2-negative, advanced breast cancer harboring a PIK3CA mutation. Here, ESR1 is linked to breast cancer.