Exon trapping was used to identify fragments of genes on human chromosome 21. One trapped sequence, hmc 18h10 (GenBank no. X88329), showed homology to a sequence (GenBank no. S65225) that includes the first three codons of the rat PEP-19 gene and 5' untranslated leader region. We have cloned the corresponding cDNA for a human homolog of the rat PEP-19 gene and mapped it to the region between markers ERG and D21S56 of chromosome 21q22.2-q22.3. Rat PEP-19 is a neuron-specific polypeptide expressed in several regions of the central nervous system. It serves as a cell-specific marker in Purkinje cells and its expression is developmentally regulated in the cerebellum, but its precise function is unknown. It is also presently unknown whether overexpression of the PEP-19 gene is involved in certain phenotypes of Down syndrome.