The fragile-X syndrome reported by Carpenter and colleagues in this issue of the Journal (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 article1 published in 1866. The cause of the disorder was established with the discovery of trisomy 21 by Lejeune et al2 almost a century later. The currently evolving complex delineation of fragile-X syndrome began with a report by Lubs3 of 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
TOWNES PL. Fragile-X Syndrome: A Jigsaw Puzzle With Picture Emerging. Am J Dis Child. 1982;136(5):389–391. doi:https://doi.org/10.1001/archpedi.1982.03970410007001
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