Fragile-X Syndrome

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Abstract
The fragile-X syndrome reported by Carpenter and colleagues in this issue of theJournal(see p 392) has recently been recognized to be a common cause of mental retardation, possibly second in frequency only to Down's syndrome (trisomy 21). Though Klinefelter's syndrome is probably more common, most persons with XXY syndrome are not intellectually handicapped. Comparison of the delineation of Down's syndrome and fragile-X syndrome discloses striking differences. Down's clinical description of "mongolian idiocy" consisted of a classic 3½-page article1published in 1866. The cause of the disorder was established with the discovery of trisomy 21 by Lejeune et al2almost a century later. The currently evolving complex delineation of fragile-X syndrome began with a report by Lubs3of a secondary constriction or "marker X" chromosome in about 10% of cells from four mentally retarded, maternally related male subjects in three generations. Only the proband had congenital