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Direct analysis of the FMR-1 gene provides an explanation for an exceptional case of a fragile X negative, mentally retarded male in a fragile X family.

Received 1 June 1992. Revised version accepted 23 July 1992. Fragile X positive males with mental retardation have been shown by many groups to have an insertion or amplification of a trinucleotide CGG repeat at the fragile X site.'-5 However, in some of these reports there have been males who appeared to be different in that they were cytogenetically fragile X negative but apparently had an insertion of DNA material and were mentally retarded. If indeed the amplification of the CGG repeat and cytogenetic expression of the fragile X site were one and the same, these males would seem to be exceptional cases. We previously reported on one such male56 who was part of an extended fragile X family (subject IV.9, fig 1). Although he had been studied cytogenetically three times under multiple media conditions, he was consistently negative. His IQ was 56 and he had always been in special education classes. Linkage analysis had indicated that he was likely to be carrying the same chromosome present in his cousins and other distant relatives affected with the fragile X syndrome. Jacobs,7 in an editorial on the fragile X syndrome, thought that this male could be explained as "carrying a small mutation that is quite independent of his phenotype". Because of the inconsistent presentation of mental retardation, apparent insertion of material at the fragile X site, and the lack of fragile X expression cytogenetically, we have studied this male further. The previously reported study involved using the available filters from the previous linkage studies.6 Following the development of other means of testing for the fragile X mutation involving a double digestion using EcoRI and EagI,8 we made fresh filters and probed them with both pOXl.95 and StB12.3.8 The results for the latter are shown in fig 2. Probe pOXi .9 gave an identical pattem. As can be seen, the male IV.9 has a complex pattern which has been termed 'mosaic'.' He clearly has a heterogeneous band indicative of an insertion or amplification of the trinucleotide CGG repeat segment at the