We have used exon trapping to contribute to the development of the transcription map of chromosome 21q22.3 and to clone the genes responsible for disorders that map in the 21q22.3 region. Polypeptides deduced from three trapped sequences that map near PFKL showed homology to the yeast PWP2 gene. The full-length coding region of a human homologue of this yeast gene was subsequently cloned from human infant brain and fetal kidney cDNA libraries. The 919-codon open reading frame of human PWP2 belongs to the family of genes that contain tryptophan-aspartate (WD) repeats; other than its yeast counterpart, PWP2 is most closely homologous to the beta subunits of the trimeric G-protein family and may putatively be involved in signal transduction. Northern blot analysis revealed that the PWP2 gene is expressed in all fetal and adult human tissues examined (3.4 kb mRNA species). This single-copy gene maps approximately 200 kb proximal to PFKL in chromosome 21q22.3 between markers EHOC-1 and D21S25.