TRINUCLEOTIDE REPEATS AT THE FRAXF LOCUS - FREQUENCY AND DISTRIBUTIONIN THE GENERAL-POPULATION

FRAXF, the third X-chromosomal fragile site to be cloned, has been sho wn to harbour a polymorphic compound triplet array: (GC-CGTC)(n) (GCC) (n), Expansion and methylation of the GCC-repeat and the neighbouring CpG-rich region result in chromosomal fragility. DNAs from 500 anonymo us consecutive newborn males were examined to determine the incidence of various repeat numbers. The range of repeats was from 10-38, with t he most common alleles having 14 (52.7%), 12 (16.6%), 21 (9.0%), and 2 2 (5.2%) triplets. Based on the distribution of repeat numbers, we sug gest that the 21-repeat allele resulted from hairpin formation involvi ng 7 GCC-repeats in a 14-repeat allele, accompanied by polymerase slip page, Examination of dinucleotide repeats near the FRAXF repeat will b e important in testing this hypothesis. Since the clinical phenotype, if any, of FRAXF is unknown, this database will also be valuable for c omparisons with repeat numbers in individuals from special populations . (C) 1996 Wiley-Liss, Inc.