In an effort to contribute to the transcript map of human chromosome 21 and the understanding of the pathophysiology of trisomy 21, we have used exon trapping to identify fragments of chromosome 21 genes. Two trapped exons, from pools of chromosome 21-specific cosmids, showed homology to the Drosophila white (w) gene. We subsequently cloned the corresponding cDNA for a human homologue of the Drosophila w gene (hW) from human retina and fetal brain cDNA libraries. The gene belongs to the ATP-binding cassette transporter gene family and is homologous to Drosophila w (and to w genes from other species) and to a lesser extent to Drosophila brown (bw) and scarlet (st) genes that are all involved in the transport of eye pigment precursor molecules. A DNA polymorphism with 62% heterozygosity due to variation of a poly (T) region in the 3' UTR of the hW has been identified and used for the incorporation of this gene to the genetic map of chromosome 21. The hW is located at 21q22.3 between DNA markers D21S212 and D21S49 in a P1 clone that also contains marker BCEI. The gene is expressed at various levels in many human tissues. The contributions of this gene to the Down syndrome phenotypes, to human eye color, and to the resulting phenotypes of null or missense mutations are presently unknown.