Trastuzumab (INN; trade name Herceptin) is a monoclonal antibody that interferes with the HER2/neu receptor. The HER receptors are proteins that are embedded in the cell membrane and communicate molecular signals from outside the cell to inside the cell, and turn genes on and off. The HER proteins regulate cell growth, survival, adhesion, migration, and differentiation—functions that are amplified or weakened in cancer cells. In some cancers, notably some breast cancers, HER2 is over-expressed, and, among other effects, causes breast cells to reproduce uncontrollably...