By using the human fetal brain library, a 1.2-kb cDNA coding for human neurogranin, a neurospecific endogenous substrate of protein kinase C, was cloned. The results of the Northern blot analysis of the human brain cortex mRNA show that the human neurogranin mRNA exists as a single transcript. Its gene thereby differs from the rat neurogranin gene (RC3), which was shown to produce two mRNAs of different lengths. cDNAs of the human and rat neurogranins show a 70% homology, whereas the protein sequences only differ in three amino acid residues out of the total of 78. The 3'-untranslated region of the human neurogranin cDNA lacks the purine-rich stretch characteristic of the rat neurogranin cDNA.