HER2 (which stands for human epidermal growth factor receptor 2) is a type of growth signal receptor, or antenna that may be present on your breast cancer cells. About 25 percent of breast cancers are HER2-positive, meaning the cancer cells make too much of a protein called HER2/neu, which indicates that the cancer may be more aggressive. If your cancer is HER2-positive, this helps doctors better predict whether the cancer may respond to certain targeted therapies.