A protein involved in normal cell growth. Human epidermal growth factor receptor 2 may be made in larger than normal amounts by some types of cancer cells, including breast, ovarian, bladder, pancreatic, and stomach cancers. This may cause cancer cells to grow more quickly and spread to other parts of the body. Checking the amount of human epidermal growth factor receptor 2 on some types of cancer cells may help plan treatment. Also called c-erbB-2, HER2, HER2/neu, and human EGF receptor 2.