||Neuregulin (NRG) has diverse functions in neural development, and one of them is the induction of acetylcholine receptors (AChRs) on the muscle fiber during the formation of neuromuscular junctions. NRG gene family includes NRG-1, NRG-2 and NRG-3. However, NRG-1 is the major form affecting the development of the nervous system. NRG-1 has two alternative splicing sites at the N-terminus; it could be an immunoglobulin (Ig)-like domain named Ig-NRG-1 or an apolar cysteine-rich domain named CRD-NRG-1. Ig-NRG-1 is expressed in neuron and muscle while CRD-NRG-1 is specifically expressed in neurons. The precise role of neuron-derived and muscle-derived NRG-1 during the early stages of development is not known. In order to study the NRG-1 expression in early embryo development, we isolated the cDNAs encoding Xenopus NRG-1 by cross-hybridization with its chick homologue. Two major isoforms, Ig-NRG-1 and CRD-NRG-1 were isolated. The amino acid sequence of Xenopus protein is 50 to 70% identical to members of the NRG-1 family. It revealed that Ig-NRG-1 and CRD-NRG-1 were generated from the same gene. cDNAs encoding different isoforms of Xenopus NRG-1s were identified, and these isoforms have two major variation sites: (i) the spacer domain with either 0 or 43 amino acids insertion, (ii) the C-terminus of EGF-like domain to derive either α or β isoform. When the EGF-like domain of Xenopus NRG-1s was expressed in mammalian cells, the recombinant protein was able to induce the expression of AChR and the tyrosine phosphorylation of ErbB receptors in cultured myotubes. Various splicing variants were expressed in different Xenopus tissues as revealed by PCR analysis. In situ hybridization showed a strong expression of NRG-1 in developing brain and spinal cord of Xenopus embryo. In addition, it was also prominently expressed in the myotomal muscle. These data suggest that in addition to motor neurons, the postsynaptic muscle cell can also contribute NRG during synaptogenesis.