Figure 2: Ligands in the EGF family interact with heteromeric or homomeric ErbB receptors in GABAergic, dopaminergic (DA), and glial cells. ErbB ligands include EGF, HB-EGF (heparin-binding EGF-like growth factor), TGFα (transforming growth factor alpha), amphiregulin, NRG1-6, and virokines (VGF, SPGF, MGF, etc.), which associate with these ErbB receptor complexes to evoke both EGF-like and NRG-like signals. ErbB1-4 selectivity of the virokines remains to be characterized. These virokines carry the EGF-like amino acid motif common to human EGF and NRG1alpha. VGF: vaccinia virus growth factor; SPGF: smallpox virus growth factor; MGF: myxoma virus growth factor.