CYP19A1 and breast cancer: The potential use of PI3K inhibitors in the post-mTORC1 inhibitor treatment setting is being investigated in BELLE-3 (NCT01633060), a placebo-controlled phase III study to investigate the safety and efficacy of buparlisib plus fulvestrant in postmenopausal women with HR-positive/HER2-negative advanced breast cancer who have received aromatase-inhibitor treatment and progressed on or after mTORC1 inhibitor-based therapy.