Fragile X syndrome (FXS) is the most commonly diagnosed inherited cause of intellectual disability. The prevalence is second only to Down syndrome as a cause of intellectual disability.It is difficult to detect Fragile X syndrome by chromosome karyotyping or aCGH. Most newborns do not show any physical signs of FXS, so the early diagnosis of FXS or carrier status is important. FXS can be diagnosed using FMR1 DNA test by polymerase chain reaction (PCR).

Fragile X syndrome (FXS) is a genetic condition caused by an abnormal expansion of CGG triplet repeat within the Fragile X mental retardation 1 (FMR1) gene, a gene located on the X chromosome. In unaffected individuals, the FMR1 gene contains 5–44 repeats of the sequence CGG. Between 45-54 repeats is considered intermediate and usually do not exhibit any symptom of FXS or FMR1-related disorders. Individuals with a premutation allele generally considered to be between 55 and 200 repeats. Individuals with fragile X syndrome have a full mutation of the FMR1 allele, with over 200 CGG repeats. The general prevalence with a full mutation is estimated at 1/3,600 males and 1/5000 females. Approximately 1/250 females and 1/800 males carry premutation alleles.

Uses polymerase chain reaction (PCR) to amplify the FMR1 gene including the CGG repeats. PCR product is analyzed by high-resolution capillary electrophoresis. Provides a reference sample which contains different CGG repeats. The reference sample can create a equation of linear regression standard curve to calculated the CGG repeats.