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In Reply: In the context of a review, I was not able to discuss many of the diseases listed in Table 2 in depth. In response to the concerns of Drs Hagerman and Hagerman, Table 2 only considered the phenotype expected for the full mutation—the one associated with the classic fragile X syndrome—and not the premutation.

Hagerman and Hagerman suggest that for carriers of the full FMR1, random X inactivation (approximately 50% normal cells) is not enough to prevent all the subtle manifestations of the full mutation. It seems that a completely normal phenotype requires some skewing in favor of the normal cells. In any case, females with a full mutation have less severe disease than males, with a significant association between the proportion of normal FMR1 alleles on the active X chromosome and IQ.^1-2 Females with the severest manifestations have the most cells expressing the mutation.³ Therefore, X . . . [Full Text of this Article]

Barbara R. Migeon, MD
bmigeon@jhmi.edu
Department of Pediatrics
Johns Hopkins University School of Medicine
Baltimore, Md

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