In order to determine the proportion of MR due to fragile X, CGG repeat analysis was carried out on DNA from 723 mentally retarded patients in S.E. Scotland. This resulted in the identification of 8 new cases of fragile X syndrome (˜1% of the total number of referrals for developmental delay). This developmentally delayed cohort was also compared to normal and high IQ cohorts to address the possibility of a relationship between CGG repeat number and IQ variation in the general population. Two complicated prenatal diagnostic cases were studied in detail to answer important practical questions concerning the nature and timing of repeat expansion. Well over 100 other XLMR conditions have been described, of which approximately 50% have been regionally mapped. Some of these are non-specific forms of XLMR in which MR is the only manifestation; others are XLMR syndromes with a characteristic and well defined phenotype. A family with non-specific XLMR is described in which the proband presented as a non dysmorphic male with unexplained MR, for exclusion of fragile X syndrome. He had a CGG allele in the normal range and the MR segregating in the family was shown not to be linked to FMRI. Subsequent analysis of polymorphic X-linked markers showed linkage to Xq21. A new XLMR syndrome is represented by a large family with severe MR and spastic diplegia which was previously thought to be MASA syndrome, a condition caused by mutations in the LICAM gene in distal Xq28. Negative LOD scores with Xq28 markers led to a search for linkage to other regions; this was found in Xp22 where several other XLMR loci are located, although the clinical features of this family differ from those of other XLMR syndromes and conditions in Xp22.