HER2 (Human epidermal factor receptor 2/neu) is a protein found on the surface of breast cells that regulates cell growth. If extra HER2 protein receptors occur on a cell due to a mutation, this over-expression of HER2 causes increased cell growth and an aggressive form of breast cancer.
HER2 positive breast cancer occurs in 20% to 30% of patients, and this form of breast cancer may not be as responsive to standard breast cancer chemotherapy, but the new monoclonal antibody medication Herceptin (transtuzumab) has been very effective in treating this particular type of breast cancer.
HER2 testing is performed on cancer cells that have been removed during breast biopsy or breast cancer surgery. It involves staining the tissue sample with a specific solution in a pathology laboratory. The pathologist then examines the cells within the tissue sample, checking for highlighted areas where high levels of HER2 overexpression are present. The test targets a gene located on chromosome 17. Depending on the level of staining, the patient’s cancer may be classified as HER2 positive or HER2 negative.