Identification of small FRAXA premutations

Background: Diagnosis of fragile X syndrome in mentally retarded individuals is satisfactorily achieved using a Southern blot test that detects the typical triplet repeat expansion (>200 repeats) within the FMR1 gene. All such individuals inherit the mutation from a carrier, who usually shows a lower triplet repeat number and may be asymptomatic. Having identified a fragile X proband, it is necessary to identify related carriers of this familial X-linked dominant mutation to provide family counseling and testing.