HER2 gene encodes a transmembrane glycoprotein with tyrosine kinase activity. It is closely related in structure to the epidermal growth factor receptor c-erbB-3/HER3/neu oncoprotein. The protein is detectable in a subset of breast and other carcinomas (urinary bladder, stomach, lung, endometrium and ovary). In the case of breast cancer, over expression determined by immunohistochemistry has been shown to be associated with poor prognosis and an indication to initiate trastuzumab/herceptin therapy.