Previously, three structurally related proteins, MyoD, myogenin, and Myf5, have been identified, and each of them was found to convert C3HlOT1/2 fibroblasts to myoblasts when their respective cDNAs were expressed under the control of a viral promoter. Here, we describe the cloning and DNA sequencing of myogenin cDNAs from chicken and mouse. They encode polypeptides highly homologous to each other, but the polypeptide sequences we have obtained are not homologous in the carboxyLterminal70 amino acids with those previously reported for mouse, rat, and human because of a single base deletion in the previously reported cDNAs. Determination of genomic sequence coding for myogenin revealed that the mouse myogenin cDNA presented here corresponds to a correct transcript of the gene, and the nucleotide sequence of the previously reported cDNA is incorrect. A comparison of chicken and mouse myogenins with other myogenic regulatory factors, MyoD and Myf-5, identified a domain with an interesting feature located in the carboxy1 terminus of these proteins in addition to the myc homology domain previously reported.