Neuregulin/Heregulin is a family of structurally related polypeptide growth factors derived from alternatively spliced genes (NRG1, NRG2, NRG3 and NRG4). To date, there are over 14 soluble and transmembrane proteins derived from the NRG1 gene. Proteolytic processing of the extracellular domain of the transmembrane NRG1 isoforms release soluble growth factors. HRG-β2 is a signaling protein for ErbB2/ErbB4 receptor on the cardiac muscle cells. The polypeptide is comprised of an immunoglobulin and epidermal growth factor-like (EGF-like) domains.