The epidermal growth factor receptor (EGFR) is a transmembrane glycoprotein that constitutes one of four members of the erbB family of tyrosine kinase receptors. The ErbB receptor tyrosine kinase family (RTK class I) consists of ErbB1/EGFR/HER1, and ErbB2-4/HER2-4. Binding of EGFR to its cognate ligands leads to autophosphorylation of receptor tyrosine kinase and subsequent activation of signal transduction pathways that are involved in regulating cellular proliferation, differentiation, and survival. Interestingly, ERbB receptor tyrosine kinases have important roles in human cancer. The gene symbol, ErbB, is derived from the name of a viral oncogene to which these receptors are homologous: Erythroblastic Leukemia Viral Oncogene. Insufficient ErbB signaling in humans is associated with the development of neurodegenerative diseases, such as multiple sclerosis and Alzheimer's disease. Additionally, research revealed that the expression or activation of epidermal growth factor receptor and ErbB2 are altered in many epithelial tumors, and clinical studies indicate that they have important roles in tumor aetiology and progression.