Direct ligand for the ERBB tyrosine kinase receptors. Induces expression of acetylcholine receptor in synaptic nuclei. Does not seem to be active. Additional isoforms seem to exist. Isoforms have alpha- or beta-type EGF-like domains. Isoform alpha1 is expressed in brain and muscle. Isoform CRD is expressed in brain and spinal cord, but at very low level in muscle. Strong expression in developing brain and spinal cord of the embryo. Also expressed in the myotomal 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.