ErbB2, is a protein that in humans is encoded by the ERBB2 gene, which is also frequently called HER2 (from human epidermal growth factor receptor 2) or CD340. HER2 contain an extracellular ligand binding domain, a transmembrane domain, and an intracellular domain that can interact with a multitude of signaling molecules and exhibit both ligand-dependent and ligand-independent activity. Amplification or overexpression of this oncogene has been shown to play an important role in the development and progression of certain aggressive types of breast cancer. In recent years the protein has become an important biomarker and target of therapy for approximately 30% of breast cancer patients.