DLTs of BYL719 in combination with T-DM1 will be assessed using National Cancer Institute's Common Toxicity Criteria for adverse events version 4.0 (except for hyperglycemia). A DLT is described any grade 3 or higher, clinically significant toxicity (excluding alopecia) experienced during the first 21 days following first dose of BYL719 that is determined to be at least possibly related to study medication. Lower grades may also be considered DLTs if they lead to a dose interruption of more than 7 consecutive days of BYL719. In general adverse events (AEs) will be graded according to the following: Grade 1 Mild AE Grade 2 Moderate AE Grade 3 Severe AE Grade 4 Life-threatening or disabling AE Grade 5 Death related to AE

Safety of BYL719 in combination with T-DM1 will be assessed during the first 21 days (1 cycle=21 days) which will assist in providing the MTD. Adverse Events including dose limiting toxicities (DLT), changes in physical findings, or clinical laboratory results as well as cardiac toxicity (changes in cardiac function, which will be measured by use of echocardiogram or MUGA scan) will be evaluated. For each dose level, 3 patients will be treated. If none (0 of 3) show a DLT, dose will be escalated for the next cohort of patients. If 2 or 3 have a DLT, then the previous dose will be considered the MTD. If 2 or 3 in cohort 1 (starting dose) experience a DLT, then the dose in -1 cohort will be used. If 1 of 3 patients has a DLT, then an additional 3 patients will be added to that dose level. If 1 of 6 has a DLT, then the dose will be escalated. If 2 of 6 have a DLT, then this dose will be considered the MTD. If 3 or more of 6 have a DLT, then the previous dose will be the MTD.