This phase II trial that includes Gemcitabine 1,250mg/m2 to be administered over approximately 30 minutes on days 1 and 8, every 21 days and Trastuzumab 8 mg/kg IV to be administered over 90 minutes on day 1 of the first cycle, then 6mg/kg as a 30 minute infusion administered on subsequent cycles.

Inclusion Criteria:

• Patients must have histologically or cytologically confirmed invasive breast cancer, with stage IV disease
• Tumors tested by IHC must be 3+ positive for HER2 overexpression. Tumors tested by FISH must be positive by the specific FISH assay for genetic amplification of HER2. Tumors that are 3+ by IHC but negative by FISH assay are ineligible
• Patients must have measurable disease
• Patients may NOT have received prior chemotherapy or prior trastuzumab therapy for metastatic breast cancer.
• Patients may have received prior chemotherapy and/or hormonal therapy for early stage breast cancer, including taxane-based chemotherapy regimens. Patients may NOT have received trastuzumab in the adjuvant setting unless disease free interval from completion of trastuzumab to time of relapse is greater than 12 months.

Exclusion Criteria:

• Patients who have had prior gemcitabine chemotherapy and adjuvant trastuzumab and had less than 12- month disease free interval
• Patients with known leptomeningeal carcinomatosis