BELLE-2 (NCT01610284) is a multicenter phase III, placebo-controlled study of buparlisib plus fulvestrant that will enroll 842 postmenopausal women with HR-positive/HER2-negative advanced breast cancer whose disease has progressed on or after aromatase-inhibitor therapy, including ≥ 334 patients with PI3K pathway alterations. The gene discussed is ERBB2; the disease is breast cancer.