Research into genetic forms of mental retardation which may account for one half of the severely retarded patients has been hampered by the enormous complexity of this condition. So far, linkage studies have been completed in 13 families and in some of them, detailed clinical studies including IQ testing and psychological examination have revealed numerous specific features that had been overlooked previously. Linkage studies in syndromic and non-syndromic cases revealed a conspicuous clustering of the underlying gene defects around the centromere and in particular, the proximal short arm of the X chromosome. This gene is highly expressed in brain and contains 26 exons.
Mutation screening in unrelated patients with X-linked mental retardation is in progress.
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