The differentiation of skeletal muscle cell includes fusion of mononucleated myoblasts into multinucleated myotubes and regulation of differentiation-related gene expression at the transcriptional, translational, and posttranslational levels. In order to study the muscle gene regulation during the differentiation of cultured chicken embryonic myoblasts, we had constructed cDNA library from myotubes and identified a developmentally regulated gene, named CHK23, from the myotube cDNA library by differential plaque hybridization. Using this cDNA clone as a probe, we observed that the amount of CHK23 transcripts is more abundant in the cell cultured in differentiation media (8102) than those cultured in growth media (811) or inhibition media (910) as judged from RNA dot blot analysis. Northern analysis showed that a 900 nucleotide transcript of CHK23 was preferentially detected in myotubes. Thus, CHK23 transcripts apparently accumulated during differentiation in vitro. The CHK23 has an open reading frame that codes for a protein of 172 amino acids. The deduced amino acid sequence from CHK23 cDNA was 88% and 90% identical to those of a growth-related tumor protein p23 in murine and human tumor cells. This striking sequence similarity among the mouse, human and chicken p23 suggests that these proteins may have very similar, if not identical, functions.