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. The HER2 protein is involved in normal cell growth. It may be made in larger than normal amounts by some types of cancer, including breast, ovarian, bladder, pancreatic, and stomach cancer. 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. It is a type of tumor marker test. Also called HER2/neu test and human epidermal growth factor receptor 2 test.