The purpose of this study is to assess the safety and efficacy of alpelisib plus fulvestrant in men and postmenopausal women with hormone receptor (HR) positive, human epidermal growth factor 2 (HER2)-negative, advanced breast cancer harboring a PIK3CA mutation in Japan, whose disease has progressed on or after aromatase inhibitor (AI) treatment regardless of prior CDK4/6 inhibitor use.