A clinical and cytogenetic study of children attending schools for the educationally handicapped has been carried out in the City of Coventry. As a result of this an estimate of the population incidence of the Martin-Bell syndrome among schoolchildren has been made. Among school-boys the incidence of mental retardation due to the fra (X) was found to be 0.73 per 1000 and among schoolgirls 0.48 per 1000. The overall prevalence is 0.61 per 1000. Genetic and cytogenetic analysis of 14 families ascertained in an unbiased manner showed that the expected 50% ratio of affected to unaffected relatives was disturbed, the possible result of a segregation distortion.

A population study has been carried out among schoolchildren in the City of Coventry in order to ascertain the frequency of mental retardation associated with the fragile X chromosome. The prevalence of the fragile X mental retardation syndrome in the 11 to 16 year age group (the age of greatest ascertainment) was about 1 0 per 1000 and therefore indicates that the syndrome is a major cause of mental retardation. Estimates of the prevalence of mental retardation in males associated with the fragile X chromosome have ranged from 0-19 to 0-92 per 10O0.'-I A study based on schools for the educationally subnormal (ESN) in Coventry has enabled us to estimate the prevalence of the fragile X chromosome in schoolchildren of that city, in both males and females. Methods The study took place between 1 April 1982 and 31 December 1985. Children were included if they were born before 31 August 1978 and if they were at school during the study period. We studied children with home addresses in Coventry who either lived at home and attended day schools for the educationally subnormal (ESN) or who were in residential care, either in or outside the city. We visited four ESN(S) and four ESN(M) schools in Coventry together with one mental deficiency hospital and two schools for the physically handicapped. We also examined at home two children who were resident during term time at schools outside Coventry. We chose to study certain groups of children among whom we anticipated that some would have the fragile X chromosome associated with their mental retardation. It was necessary to omit certain groups in whom the fragile X chromosome was less likely to be found because the techniques required to identify the fragile X chromosome are laborious4 5

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The superconducting oxide spinel system Li1+xTi2-xO4 (00 has been achieved via dilution of the 7Li nuclei with 6Li. NMR linewidths show a narrowing with increasing temperature for low x, indicating that the Li ions are becoming mobile around room temperature. Observed chemical shifts show an increase at the metal-insulator transition, demonstrating that the Li nuclei act as weakly coupled probes of the bulk paramagnetic susceptibility. Analysis of these shifts, together with nuclear spin-lattice relaxation times, has allowed the Knight shift due to the Li(2s) contact to be calculated. This reveals that there is approximately 7% Li 2s character in the unpaired-electron wave function at the Fermi surface in LiTi2O4.

SUMMARY A genetic study of children attending ESN(M) schools in Coventry has shown a recurrence risk of idiopathic mental retardation in sibs lying between 1 in 4 and 1 in 5. There was also a prevalence of mental retardation in other relatives that was greater than the population prevalence, and was less for second degree relatives than for first degree, and less still for third degree relatives. Recurrence in sibs was greater if more than one first degree relative was affected. There was no suggestion of a contribution by X linked genes, once the fragile X syndrome had been excluded. The presence of perinatal and other environmental factors in the index children did not alter the recurrence risk for sibs except for very low birth weight. There was a low recurrence rate of mental retardation in Asian families, suggesting that they had a different distribution of intelligence from non-Asian families.