HER2 is a gene that provides instructions for making a protein called human epidermal growth factor receptor 2. This protein is found on the surface of some cells and helps regulate cell growth and division. In certain types of cancer, particularly breast cancer, the HER2 gene can become amplified, leading to overproduction of the HER2 protein. This can cause cells to grow and divide uncontrollably. Testing for HER2 status is important in cancer treatment, as it helps determine the most effective therapies. Patients with HER2-positive cancers may benefit from targeted treatments, such as trastuzumab (Herceptin), which specifically attack cells with high levels of the HER2 protein.