HER2 stands for human epidermal growth factor receptor. It is a protein found in small amounts on some normal cells, including breast cells, stomach cells and bladder cells. It is one of the proteins involved in cell growth. Some cancers have cells with large amounts of this protein and they are called HER2 positive. These cancers can be treated with drugs that target the HER2 protein. If a cancer does not have large amounts of the HER2 protein it is called HER2 negative.