Direct ligand for the ERBB tyrosine kinase receptors. The multiple isoforms perform diverse functions: cysteine-rich domain containing isoforms (isoform 2-isoform 4) probably regulate the expression of nicotinic acetylcholine receptors at developing interneuronal synapses. Isoform Ig-NRG is required for the initial induction and/or maintenance of the mature levels of acetylcholine receptors at neuromuscular synapses (PubMed:8453670, PubMed:9491987). Binds to ERBB3 and integrins to form a complex which is essential for NRG1-ERBB signaling (By similarity). Does not seem to be active. Additional isoforms seem to exist. Isoform 2-isoform 4 are detected at embryonic day 4 (ED4) in both visceral and somatic motor neurons of spinal cord and is highest at ED6. Isoform 1 is not expressed until ED 6 in spinal cord. At ED 11 both isoforms display comparable levels. 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. Contains an Ig-like domain. The EGF-like domain is replaced by a cysteine-rich domain (CRD). The EGF-like domain is replaced by a cysteine-rich domain (CRD). The EGF-like domain is replaced by a cysteine-rich domain (CRD). Belongs to the neuregulin family.