The fragile-X syndrome, IV. Progress towards the identification of linked restriction fragment length variants...

We present our strategy and progress towards the identification of rare restriction fragment length variants (RFLVs) segregating with the fragile-X syndrome. DNA from a carrier mother and two retarded sons was digested with 7 restriction endonucleases and Southern blots were probed with cloned unique X-chromosomal sequences. Two of 17 cloned segments tested revealed RFLVs between the X-chromosomes of the carrier mother. One of them detected variants using Pvu II and Msp I. The Pvu II and Msp I alleles found on the X-chromosome bearing the fragile-X mutation were not found in 31 and 22 random X-chromosomes, respectively. The other probe detected variants using Pvu II and Taq I. The Taq I allele present on the X-chromosome with the fragile-X mutation was found in 23 out of 25 random X-chromosomes, while the Pvu II allele was not found in 21 random X-chromosomes. One of these probes and two other cloned unique X-chromosome sequences were localized distal to Xq26 by in situ hybridization to prometaphase chromosomes and by probing Southern blots containing DNA from a deleted X-chromosome. These are being used for linkage analysis in an extended family.