This is a phase II, exploratory, open-label, single arm study of BYL719 monotherapy, a selective phosphatidylinositol 3-kinase (PI3K) alpha inhibitor, in adult patients with advanced metastatic breast cancer progressing after first line therapy. Patients with advanced hormone receptor positive tumors will be required to have an alteration of the PI3K pathway. Those patients with advanced triple negative breast cancers are genetically unselected for this study.