Test Details: Fragile X syndrome is due to an expansion of an unstable CGG repeat sequence located in the 5’ untranslated region of the FMR1 gene on chromosome X. When the CGG repeat number (n) is greater than 200, it is defined as a full mutation of the FMR1 gene. CGG repeat number of FMR1 gene is determined by quantitative PCR followed by capillary electrophoresis.