Fragile X-Syndrome is the most common inherited form of mental retardation. The disease is caused by defect of the fragile X mental retardation gene (FMR 1), located on X chromosome, due to expansion of the repetitive CGG sequence in the promoter region. The modern approach for diagnosis of the disease is based on the use of direct DNA analysis of the FMR 1 gene. Recombinant DNA technology techniques overcome the deficiencies of the cyto-genetic test, such as large number of false positive results.In the molecular diagnostics of Fragile X-Syndrome, two basic DNA methods are used: a) Southern blot analysis of Eco RI or Eco R I/Eag I digested DNA samples and b) PCR for amplification of CG rich regions. Southern blot is used for detection of full mutations and premutations, and PCR for normal and premutation alleles sizing.