Exon trapping/amplification was used to clone portions of genes from the Down syndrome critical region (DSCR) of human chromosome 21q22. Two trapped sequences showed complete homology with nucleotide sequence D23672 of Genbank which corresponds to the gene for the human holocarboxylase synthetase (HCS) that was previously assigned to chromosome 21. We precisely mapped this gene to the DSCR by somatic cell hybrids, chromosome 21-specific YACs, and hybridization to chromosome 21-specific cosmids; it localizes to YACs 745H11 and 230E8 of the Chumakov et al. (Nature 359:380, 1992) YAC contig in a region of less that one megabase between markers D21S333 and D21S267. This HCS gene may contribute in a gene dosage-dependent manner to the phenotype of Down syndrome.