A laboratory test that measures the amount of HER2 protein on cancer cells or how many copies of the HER2 gene are in the DNA of cancer cells. The HER2 protein helps control normal cell growth. Larger than normal amounts of the HER2 protein or too many copies of the HER2 gene may be made by some types of cancer, 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. A HER2 test may be done to help plan treatment, which may include drugs that target the HER2 protein. It is a type of tumor marker test. Also called HER2/neu test and human epidermal growth factor receptor 2 test.