ErbB2 is encoded by the ERBB2 gene in humans, commonly referred to as the oncogene human epidermal growth factor receptor 2 (HER2) or CD340. HER2 is composed of three parts: extracellular ligand binding domain, transmembrane domain, and intracellular domain. These domains can interact with a large number of signaling molecules, demonstrating ligand dependent and non ligand dependent activity. Amplification or overexpression of the oncogene plays an important role in the development of some undesirable types of breast cancer. In recent years, this protein has become an important target of biological therapy for nearly 30% of breast cancer patients.