Xq28 has been of special Interest in human genetics because a large number of diseases map to this region. As a step In the molecular analysis of the as yet uncloned disease genes, and as a test for the detailed analysis of larger regions of the genome, we have constructed YAC clone contlgs covering the 7.5 Mb region between IDS to the telomere on the long arm of the human X chromosome. Contlgs were assembled and verified by an Integrated hybridization-based strategy. Data was combined from the physical map, from YAC and cosmld mapping experiments, and from the localization of specific transcripts In the region. Two gaps in the YAC map of 250 and 100 kb were covered In part by the aid of cosmid clones, but small gaps of 50 kb each remain. The cloned region is expected to contain yet unidentified genes for at least ten genetic diseases. The construction of ordered YAC clone contlgs of Xq28 represents an Important step In the molecular Identification of these genes, and the further analysis of one of the genetically most Interesting regions of the human genome.