HER2 (human epidermal growth factor) is a protein that can affect the growth of some cancer cells. It is found on the surface of normal breast cells. Some breast cancer cells have a very high number of HER2 receptors. The extra HER2 receptors stimulate the cancer cells to divide and grow. When there are higher levels of the HER2 protein in a breast cancer, it is called HER2 positive breast cancer.
The higher the level of HER2, the more likely the cell is to grow and divide. Between 15 and 25 out of every 100 women with breast cancer (15 to 25%) have HER2 positive cancers. Fewer men with breast cancer are thought to have HER2 positive cancers.
HER2 positive breast cancers tend to grow more quickly than HER2 negative breast cancers. However, effective treatments called targeted (biological) therapies have been developed to treat HER2 positive breast cancer. The drug most commonly used is trastuzumab (Herceptin®).