A drug used to treat HER2-positive breast cancer. It is also used with other drugs to treat HER2-positive stomach cancer or HER2-positive gastroesophageal junction cancer that has spread to other parts of the body and has not already been treated. It is also being studied in the treatment of other types of cancer. Herceptin binds to a protein called HER2, which is found on some cancer cells. This may help the immune system kill cancer cells. Herceptin contains the active ingredient trastuzumab. It is a type of monoclonal antibody and a type of HER2 receptor antagonist.