ErbB4, also known as Her4, is a member of the tyrosine kinase receptors (RTK) and the epidermal growth factor receptor (EGFR) subfamily, and appears to play important roles in differentiation, development and certain carcinomas. It is a single transmembrane-span type I glycoprotein structurally consisting of multiple cysteine rich domains, a transmembrane domain, a tyrosine kinase domain, a phosphotidylinositol-3 (PI-3) kinase binding site and a PDZ domain binding motif. However, the last two domains are absent in one of the two naturally occurring ErbB4 isoforms, ErbB4 CYT-2. ErbB4 ligands include the neuregulins, betacellulin and heparin-binding EGF-like growth factor (HB-EGF). Typically, heterodimerization of ErbB4 with ErbB2 forms the h