Panels of somatic cell hybrid lines carrying various structural rearrangements of the human X chromosome short arm were analyzed with 21 X-chromosome-specific cloned DNA fragments. We mapped these molecular markers to five different regions of the short arm of the X chromosome. The results were confirmed by gene-dosage studies of human lymphoblasts with structurally abnormal X chromosomes. The ornithine transcarbamylase gene and four anonymous DNA sequences map within band Xp21, flanking the presumed locus for Duchenne muscular dystrophy.