While performing a systematic search for chromosomal microdeletions in patients with clinically complex X-linked syndromes, we have observed that large male-viable deletions and duplications are clustered in heterochromatic regions of the X chromosome. Apart from the Xp21 band, where numerous deletions have been found that encompass the Duchenne muscular dystrophy gene, an increasing number of deletions and duplications have been observed that span (part of) the Xq21 segment. To refine the molecular and genetic map of this region, we have employed 52 cloned single-copy DNA sequences from the Xcen-q22 segment to characterize two partly overlapping tandem duplications and two interstitial deletions on the proximal long arm of the human X chromosome. Together with a panel of somatic cell hybrids that had been described earlier, these four rearrangements enabled us to order the 52 probes into nine different groups and to narrow the regional assignment of several genes, including those for tapetochoroidal dystrophy and anhidrotic ectodermal dysplasia.