Background: Despite the availability of multiple effective therapies, most patients with metastatic HER2-positive breast cancer will experience disease progression and death. While traditionally the focus has been targeting the HER receptor family itself, combinations involving targets downstream of the HER2 pathway, particularly CDK 4/6, could potentially enhance therapeutic efficacy. In pre-clinical models of acquired resistance to HER2-targeted therapies, inhibition of CDK4/6 has been shown to result in tumor inhibition. Trial Design: This phase Ib, single arm, open-label clinical trialis investigating the combination of Trastuzumab emtansine (T-DM1) and the CDK4/6 inhibitor, ribociclib (LEE011). Eligible patients include patients age ≥ 18 years with HER2-positive metastatic breast cancer. Prior treatment with at least one regimen containing trastuzumab and a taxane is required. Ribociclib is given orally for two weeks of a 21-day cycle (days 8-21), with T-DM1 given at standard dose every 3 weeks on day 1. Trial Objectives: 1. To estimate the MTD and/or RP2D of ribociclib in combination with T-DM1. 2. To assess the safety and tolerability of ribociclib in combination with T-DM1. 3. To explore the clinical activity of T-DM1 and ribociclib in HER2-positive metastatic breast cancer. 4. To assess potential biomarkers of response to ribociclib in combination with T-DM1. Statistical Methods: A standard 3+3 dose escalation design is being used to evaluate various doses of ribociclib in combination with T-DM1 to determine the maximum tolerated dose (MTD) and/or recommended phase-2 dose (RP2D). Once MTD/RP2D is determined there will be a dose-expansion cohort (N = l5) to confirm the safety profile and evaluate preliminary evidence of efficacy, including objective response rate (ORR) by RECIST 1.1 and progression-free survival (PFS). Clinical trial information: NCT02657343