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The Fragile X Prevalence Paradox
  1. Paul J. Hagerman (pjhagerman{at}
  1. University of California Davis School of Medicine, United States


    Although fragile X syndrome (FXS; OMIM #300624) is generally regarded as the most common inherited form of cognitive impairment 1-3, there is little consensus as to its prevalence in the general population or to sex-specific differences in prevalence. Estimates of FXS prevalence (~1/4,000 - 1/8,000) that are based on population projections from cohorts of children with special education needs (SEN) generally underestimate the extent of clinical involvement (comprehensive summary: 4), since many individuals affected by the behavioral, emotional, and/or learning disabilities of FXS have IQs in the normal or borderline range. 5, 6 The latter individuals may not be included in cohorts that use cognitive impairment as an inclusion criterion, a problem that is particularly marked for girls, with the majority having IQs within the normal range.7

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