The neuregulin (also known as heregulin) cytokine family is comprised of four genes that encode a large number of secreted or membrane-bound isoforms. All family members share an EGF-like domain that interacts with the ErbB family of tyrosine kinase receptors. The NRG1 gene is the most pleiotropic, encoding more than 14 soluble or transmembrane proteins. NRG1 isoforms can be classified into type I (neu differentiation factor, NDF; heregulin, HRG; acetylcholine receptor inducing activity, ARIA), type II (glial growth factor, GGF) and type III (sensory and motor neuron-derived factor, SMDF) isoforms. NRG proteins show distinct spatial and temporal expression patterns and play important roles during development of both the nervous system and the heart.