Mouse P19 embryonal carcinoma cells can be reproducibly differentiated into neurons and glial cells upon treatment with high concentration of retinoic acid (RA). In order to understand the molecular mechanisms that control early neural differentiation, we screened a cDNA library made from 24-h RA-treated P19 cells with subtracted cDNA probes. One clone was positive in the secondary screening and was designated as p205. This clone (1.1 kb) has an open reading frame of 317 amino acids with homology to G-protein beta subunit. This protein sequence was identical to chicken and human genes previously identified as a major histocompatibility complex-associated gene. The complete conservation of its amino acid sequence between mouse, human and chicken provides strong evidence that the p205 protein fulfills a fundamental function. Developmental Northern blot analysis revealed that a p205 mRNA is expressed at high levels in the embryonic mouse brain, decreasing as development proceeds. In situ hybridization revealed that p205 mRNA is strongly and ubiquitously expressed in the embryonic and early postnatal mouse brain. This expression decreased during postnatal development and was localized in the dentate gyrus, habenula, piriform cortex, paraventricular nucleus of the hypothalamus and supraoptic nucleus of the adult brain. These results suggest that this protein plays an important role in the developing brain and neuronal differentiation.