Several neuron-specific cDNAs were identified using large-scale collection of 3'-directed partial cDNA sequences from a human neuroblastoma cell line. One of such cDNAs encoded a protein homologous to the mouse NeuroD. The mouse NeuroD, a basic helix-loop-helix (bHLH) protein seems to function as a differentiation factor for neurogenesis, as the gene is transiently expressed in postmitoic differentiating neurons. The human counterpart has a bHLH domain completely identical to that of the mouse NeuroD. But its expression was not only in the fetal brain but also in the adult cerebellum. The result of cross-species in situ hybridization also showed the transcripts were detected in the granule cell layer of the adult mouse cerebellum. The results suggest that the human as well as the mouse neuroD may play some important functions not only in the fetal brain but also in the fetal brain but also in the matured neurons of the cerebellum.