HER2 testing looks at a sample of tumor tissue. The most common ways to test tumor tissue are: Immunohistochemistry (IHC) testing measures the HER2 protein on the surface of the cells. Fluorescence in situ hybridization (FISH) testing looks for extra copies of the HER2 gene.HER2 status testing is done on a biopsy sample taken from the tumour. There are 2 ways to test HER2 status. Immunohistochemistry (IHC) measures the amount of HER2 protein in the cancer cells. Fluorescence in situ hybridization (FISH) looks at the number of copies of the HER2 gene in the cancer cells.When a cancer is HER2-positive, it means that the cancer cells make too much HER2 protein, which can cause tumors to grow more rapidly than with other forms of breast cancer. Drugs that target the HER2 proteins are the primary treatment for this type of breast cancer, given along with chemotherapy.Apr 15, 2019While overall the prognosis of HER2-positive tumors tends to be somewhat poorer than for those that are estrogen receptor-positive but HER2-negative, the widespread adoption of HER2 therapies is making a difference in survival rates, as well as reduced risk for recurrence.