On Apr. 27, 2005, results from two large National Cancer Institute sponsored randomized clinical trials showed that patients with early-stage, HER2-positive, invasive breast cancer who received the monoclonal antibody trastuzumab (Herceptin) in combination with chemotherapy experienced a significant decrease in their risk of cancer recurrence in comparison with patients who received the same chemotherapy without trastuzumab.

Trastuzumab slows or stops the growth of these cells, and it is only used to treat cancers that overexpress the HER–2 protein. Approximately 20 percent to 30 percent of breast cancers overexpress HER-2. These tumors tend to grow faster and are generally more likely to recur than tumors that do not overproduce HER-2.