A drug used to treat breast cancer that is HER2-positive (expresses the human epidermal growth factor receptor 2). It is also used with other drugs to treat HER2-positive stomach cancer that has not already been treated and has spread to other parts of the body. It is being studied in the treatment of other types of cancer. Trastuzumab binds to HER2 on the surface of HER2-positive cancer cells, and may kill them. It is a type of monoclonal antibody. Also called Herceptin.