Low affinity ligand for the ERBB4 tyrosine kinase receptor. Concomitantly recruits ERBB1 and ERBB2 coreceptors, resulting in ligand-stimulated tyrosine phosphorylation and activation of the ERBB receptors. Does not bind to the ERBB1, ERBB2 and ERBB3 receptors. Interacts with ERBB4. Does not seem to be active. At least 3 isoforms may be produced. Highly expressed in pancreas; weakly expressed in muscle. The cytoplasmic domain may be involved in the regulation of trafficking and proteolytic processing. Regulation of the proteolytic processing involves initial intracellular domain dimerization (By similarity). ERBB receptor binding is elicited entirely by the EGF-like domain. Proteolytic cleavage close to the plasma membrane on the external face leads to the release of the soluble growth factor form. Extensive glycosylation precedes the proteolytic cleavage. Belongs to the neuregulin family.