Haplotype study of intermediate-length alleles at the fragile X (FMR1) gene: ATL1, FMRb, and microsatellite haplotypes differ from those found in common-size FMR1 alleles.

The CGG repeat within the X-chromosome-linked FMR1 gene, which in hyperexpansion (> 200 copies) results in fragile X syndrome, is highly polymorphic. The mechanism of expansion is not well understood, but CGG repeats called intermediate-length or gray zone alleles (approximately equal 35-60 repeats) are thought to make up the FMR1 alleles showing initial steps in this expansion process. It has been hypothesized that the background haplotype of these alleles plays a role in their susceptibility to expansion.