The ErbB or EGF family of transmembrane RTKs plays an important role during growth and development of a number of tissues including the heart, mammary gland and the central nervous system. The ErbB family includes four members: EGFR/ErbB1/Her1, ErbB2/Her2, ErbB3/Her3, and ErbB4/Her4. ErbBs consist of an extracellular ligand-binding domain, a single membrane-spanning region and a cytoplasmic protein tyrosine kinase domain. Although these receptors share common structural elements, ligands have been identified only for ErbB1, ErbB3, and ErbB4. ErbB2 remains an orphan receptor with no diffusible ErbB2 specific ligand identified...