Additional Notes

Direct DNA analysis of the CGG repeat in the fragile X mental retardation 1 gene (FMR1) is performed by Triplet Repeat Primed PCR (Asuragen AmplidexTM) followed by capillary electrophoresis.

Normal alleles, which have no meiotic or mitotic instability, contain up to 44 copies of the CGG repeat in the 5’ untranslated region of the FMR1 gene.  Abnormal alleles contain dynamic mutations that are classified as intermediate, premutation, or full mutation based on the number of trinucleotide repeats. Intermediate or grey zone alleles contain 45-54 CGG repeats. Approximately 14% of intermediate alleles are unstable and may expand into the premutation range, but not to full mutation range, when transmitted by the mother. Premutation alleles contain between 55-200 CGG repeats and possess an increased potential for expansion to a full mutation during transmission. Full mutations are characterized by the presence of 200-230 to several thousand CGG repeats, and are associated with intellectual disability in affected males and mild intellectual disability in affected females. The FMR1 mutation analysis as performed here is greater than 99% accurate and identifies at least 99% of all fragile X mutations.