HER2 (Human Epidermal Growth Factor Receptor 2) is a gene that provides instructions for the synthesis of a protein receptor found on the surface of certain cells, including breast cells. This receptor can receive signals that stimulate cell growth. An abnormal increase in the number of copies of the HER2 gene in a cell is known as HER2 amplification. When the HER2 gene is amplified, the cell produces too many HER2 receptors, leading to uncontrolled cell growth and division.

HER2 amplification is particularly relevant in the context of breast cancer, but it can also occur in other cancers such as gastric cancer. HER2 amplification occurs in about 15-20% of breast cancers. It is associated with a more aggressive form of the disease and an increased risk of recurrence. However, the development of targeted therapies, such as trastuzumab (Herceptin) and other anti-HER2 drugs, has significantly improved the treatment outcomes for patients with HER2-positive breast cancer.