This is a lab test that measures the amount of a certain gene in cells. It can be used to see if an invasive cancer has too many HER2 genes. A cancer with too many of these genes is called HER2-positive. If a cancer is HER2-positive, it may respond to Herceptin (chemical name: trastuzumab). This is a drug that can slow down or stop the growth of cancer with too many HER2 genes.
Read more about fluorescence in situ hybridization (FISH) test.