Human epidermal growth factor receptor-2 (HER2/erbB-2) is a member of a family of four transmembrane receptor tyrosine kinases that regulate cell growth, survival and differentiation via multiple signal transduction pathways. Amplification of the HER2 gene occurs in 20-25% of human breast cancers. This amplification event is an independent adverse prognostic factor as well as a predictive factor for increased response to doxorubicin-based combination chemotherapy, response to trastuzumab and decreased response to hormonal therapy. Methods for detecting protein overexpression or gene amplification in clinical tumor specimens include immunohistochemistry (IHC) and fluorescence in situ hybridization (FISH) techniques, with the latter considered by some to be more accurate. Trastuzumab (Herceptin) is a recombinant humanized monoclonal antibody which targets an epitope in the extracellular domain of the HER2 protein. Preclinical models demonstrated that this antibody has significant anti-tumor activity as a single agent and has synergy with certain chemotherapeutic drugs. Phase II and III clinical trials performed in women with metastatic breast cancer that overexpress HER2 have shown that trastuzumab has clinical activity when used as first-, second- or third-line monotherapy, and improves survival when used as first-line therapy in combination with chemotherapy. Newer combinations with numerous chemotherapeutic drugs have also shown significant clinical activity in phase II studies. In all of these trials, trastuzumab was generally well-tolerated, but cardiac toxicity (particularly when the antibody was combined with anthracyclines) was an unexpected adverse effect. Although trastuzumab is currently usually administered on a weekly intravenous schedule, evidence suggests that a triple dose of the drug given once every three weeks has a pharmacokinetic profile expected to be equally efficacious. Neither the optimal schedule nor the optimal duration of trastuzumab therapy has yet been clearly defined in controlled clinical trials. Current clinical investigations of trastuzumab include its use in both the adjuvant and neoadjuvant settings as well as in combination with other chemotherapy drugs or new biologic targeted agents.