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. The alpha-type and beta-type differ in the EGF-like domain. Expressed in most parts of the brain, especially the olfactory bulb and cerebellum where it localizes in granule and Purkinje cells. In the hippocampus, found in the granule cells of the dentate gyrus. In the basal forebrain, found in the cholinergic cells. In the hindbrain, weakly detectable in the motor trigeminal nucleus. Not detected in the hypothalamus. Also found in the liver and in the thymus. Not detected in heart, adrenal gland, or testis. In the embryo, expressed in the brain of E11.5 embryos where it is found in the telencephalon, but not in the hindbrain. Not found in the heart. In the adult, found in brain and thymus. 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.