Neuregulin 1 (NRG1, isoform HRG-beta2) belongs to the neuregulin/heregulin cytokine family consisting of variants of the four genes NRG1, NRG2, NRG3 and NRG4. More than 15 transmembrane and secreted protein isoforms exist for NRG1, they are generated by alternative promoter usage and mRNA splicing. The different isoforms all contain immunoglobulin (Ig) and epidermal growth factor-like (EGF-like) domains. The EGF-like domain interacts with ErbB3 and ErbB4 tyrosine kinase receptors, resulting in tyrosine phosphorylation and NRG1 signaling. NRG proteins show distinct spatial and temporal expression patterns. These proteins are important for the development of the nervous system and the heart and in the regulation of neurotransmission.