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