Studies on the population genetics of fragile X syndrome have relied on haplotyping microsatellite markers closely flanking the FMR1 gene on normal and fragile X chromosomes. Strong linkage disequilibrium between the fragile X mutation and a few haplotypes have been reported in several populations, suggesting the occurrence of a limited number of initial mutational events [Chiurazzi et al., 1996a]. The three microsatellite repeats most frequently employed in haplotyping are DXS548 [Riggins et al., 1992], FRAXAC1, and FRAXAC2 [Richards et al., 1991]. Different systems have been used by various groups for identifying the numerous alleles at each of these marker loci, thus impeding reliable comparisons of the results across populations and conclusions on the identity of fragile X "founder chromosomes." In fact, several authors attempted to align the allele distributions of the flanking microsatellites by assuming that the mode(s) in populations of European origin were the same [Chiurazzi et al., 1996a;Macpherson et al., 1994;Rousseau et al., 1995]. However, this "alignment" procedure of different distributions cannot be considered satisfactory and an absolute reference must be introduced at some point. Unfortunately, sizing of polymerase chain reaction (PCR) products by running them next to markers or even sequences (e.g., M13) derived from other loci is not a reliable method at the single base-pair resolution level, because the different GC content and secondary structures make fragments of the same length migrate with slight differences. Thus, only one (or more) allele(s) of the same microsatellite which have been actually sequenced would constitute such an absolute reference. However, sequence information for all three loci obtained from the same samples is not yet available, and considering that sequencing of repeated sequences can be troublesome and that different centers may obtain different results, we first decided to genotype a few lymphoblastoid cell lines for these markers and to start distributing their DNA to all interested laboratories. Therefore, we have established a panel of DNA samples derived from several cell lines maintained in our laboratories and that we are intending to sequence. It is important to realize that, even in the absence of sequence information, the sharing of reference DNAs will allow comparison of the results obtained in the various FMR1 haplotype studies. All DNA samples (seven males and one female for a total of eight) were genotyped for DXS548, FRAXAC1, and FRAXAC2 independently by each participating group in Italy, the United Kingdom, and Canada, respectively. This letter to the editor is intended to call the attention of fellow researchers who wish to replicate our sequencing effort and to inform them that this panel is available freely to all researchers and can be requested at the Internet address http://www.rmga.qc. ca/panel/. Upon request via the WWW site, the closest reference laboratory will provide two or three representative DNA samples from the panel. Ideally, all ...
