We isolated several cDNA fragments by immunoscreening a human cDNA library with our monoclonal antibody, BG5, that showed neuronal staining on human and rat brain sections. A 1,570 bp sequence of one cDNA fragment showed 75% homology to the rat microtubule-associated protein 1A (MAP1A) cDNA sequence. This rat MAP1A-like human cDNA was highly specific to the adult brain among human tissues tested, and was expressed in various brain regions including white matter. The size of the mRNA detected with Northern blot analysis in adult human brain equaled 10 kb. The gene of this cDNA was assigned to human chromosome 15 that has a syntenic region of mouse chromosome 2, where the mouse MAP1A gene has been assigned. These results indicate that this rat MAP1A-like cDNA is a portion of human MAP1A and is a conserved molecular species among humans and rodents.