Fragile X syndrome is characterized in affected hemizygotes by severe mental retardation, hyperkinetic behavior, problém wiťh concentration and infantile autism, delayed development of speech, macrocephaly, large ears, protruding jaw and chin, high-pitched voice and postpubertal macroorchidism. Name of this affection resem-bles a cytogenetic marker - fragile site on the long arm of chromosome X in region Xq27 - folie acid depended. This phenotype is associated with mutation in FMR1 gene and clinical as well as laboratory findings show very variable expression due to different size of amplification of CGG repeats, somatic mosaicism and parental origin of the mutation. Population incidence of this disorder is 1:3000 and it is second most common cause of mental retardation in man. Amplification of CGG tri-nucleotide repeats 50-200 is unstable and represents premutation, which leads in hemizygotes to the normál transmitting males (NTM) who are carriers of gonosomal recessive disorder. Authors refer the familial oceurrence of fragile X syndrome, 4 persons with full mutation and severe mental deficiency in 3 males and borderline intelligence in their mother and 2 cases of fragile X premutation in both graduated persons, but with progressive intention tremor, Parkinsonism in the man and premature ovarian failure in the woman.

        Key words: fragile X syndrome, gonosomal recessive inheritance, mental retardation, somatic mosaicism, premutation, normál transmitting male, progressive ataxia, parkinsonism, premature ovarian failure