Background

HER2 is a receptor (specialized protein) on the surface of breast cells and breast cancer cells that controls cancer cell growth, invasion, and spread of the cancer to other parts of the body. Every healthy breast cell has two copies of the HER2 gene, which controls production of the HER2 protein. High levels of the HER2 gene and/or protein occur in approximately 20% of patients with breast cancer. These tumors, referred to as HER2-positive, tend to be higher risk cancers, and often respond differently to some types of treatment, such as certain types of chemotherapy or hormone treatments.

A new class of drugs specifically blocks HER2 to stop the growth of cancer cells. One of these drugs, trastuzumab (Herceptin), is approved by the U.S. Food and Drug Administration (FDA) to treat HER2-positive breast cancer. Trastuzumab has shown to be effective against HER2-positive cancers. However, this drug is costly and causes serious side effects, especially heart weakness, in a small number of patients. Therefore, it is important to accurately determine the HER2 status of each patient's breast cancer to make sure that the patients most likely to be helped (those with HER2-positive cancer) are offered the drug, and those unlikely to be helped (those with HER2-negative cancer) can avoid potential side effects.

The two FDA-approved methods currently used in the United States to test for HER2 are immunohistochemistry (IHC) and fluorescence in-situ hybridization (FISH). IHC testing can show how much of the HER2 protein is present on the tumor cell surface, while FISH testing measures the number of copies of the HER2 gene inside each cell.