Direct ligand for ERBB3 and ERBB4 tyrosine kinase receptors. Concomitantly recruits ERBB1 and ERBB2 coreceptors, resulting in ligand-stimulated tyrosine phosphorylation and activation of the ERBB receptors. May also promote the heterodimerization with the EGF receptor. Interacts with ERBB3 and ERBB4. Does not seem to be active. Additional isoforms seem to exist. Highest expression in the brain, with lower levels in the lung. In the cerebellum, found in granule and Purkinje cells. 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.