 This is a phase I, multicenter, open-label, dose-escalation study of single-agent trastuzumab-MCC-DM1 administered by intravenous IV infusion in patients with HER2-positive metastatic breast cancer MBC who have previously received trastuzumab. The study will assess the safety, tolerability, and pharmacokinetics of trastuzumab-MCC-DM1 and determine the dose and schedule to be used in Phase II. 