The HER2 gene codes for a receptor on the surface of some cancer cells, human epidermal growth factor receptor 2.These cancer cells may contain more than the usual two copies of the HER2 gene which can lead to overproduction of the receptors that are displayed on the cell's surface.Excess presence of these receptors leads to a more aggressive form of breast cancer that grows more quickly than other forms of cancer. HER2 positivity is indicated if there are more than six copies of the HER2 gene per cell or if more than 2.2 HER2 genes are found for each copy of chromosome 17. 2  The latter is termed the HER2 chromosome enumeration probe 17 or the CEP17 ratio.