Necdin is a polypeptide sequence encoded by neural differentiation-specific mRNA derived from embryonal carcinoma cells. We have examined the expression of necdin and its mRNA in cultured cells and mouse brain by Northern blot analysis and immunohistochemistry. Among various established cell lines including neuroblastoma and glioma cells, only differentiated embryonal carcinoma cells (P19 and F9) expressed necdin mRNA. Necdin immunoreactivity was localized in the nuclei of differentiated neurons derived from P19 cells. Necdin mRNA was detected throughout brain regions of adult mouse; the relative abundances in the hypothalamus and midbrain were the highest, whereas those in the olfactory bulb and cerebellum were the lowest. In developing mouse brain, necdin mRNA was expressed during early periods of neuronal generation and differentiation, and the peak levels were attained during postnatal days 1-4. Necdin immunoreactivity was not detected in the neural stem cells on embryonic day 10, but was concentrated in the nuclei of brain cells, mostly neurons, at advanced stages of differentiation. The majority of differentiated neurons in the brain had necdin-immunoreactive nuclei on postnatal day 33. Thus, necdin may represent a valuable molecular marker for differentiated neurons both in vitro and in vivo.