The proto-oncogene human epidermal growth factor receptor 2 (HER2) gene also known as ERBB2 is a member of the ERBB/HER RTK family which is comprised of EGFR (EGFR/HER1/ERBB1), HER3/ERBB3, and HER4/ERBB4. This gene encodes for a transmembrane glycoprotein receptor with an intracellular domain with tyrosine kinase activity which is responsible for the activation of intracellular signal transduction pathways controlling epithelial cell growth, differentiation, and angiogenesis. Aberrations of the HER2 gene which usually leads to increased receptor signaling have been observed in many different types of cancer.