The term “fragile X syndrome” is named after a constriction recognizable on chromosome (at Xq 27.3) cultured at chromosome media without folic acid. The unstable part includes repetition of 3 nucleotides which is intensified at subsequent generation (DNA amplification) and gives rise to a more severe phenotype in the individual. About 20% of males have normal fragile X syndrome. The daughters of these individuals have abnormal chromosomes (carrier) and their grandchildren will be marked. In typical syndrome, the boys will suffer from mental retardation, macrocephalia, large or protruding ears, elongated face, and macroorchidism.

In terms of behavioral comorbidity, symptoms are similar to pervasive developmental disorder such as autism and attention deficit/hyperactivity disorder. The impairments in cognitive abilities are manifested as learning difficulties to severe problems.

Our patients were 2 boys (6 and 7 years old) referred due to their hyperactivity. In physical examination, attention deficit/hyperactivity disorder as well as fragile X syndrome were confirmed. In chromosome culture test, the constriction at Xq 27.3 was specified.