Infantile autism: an occasional manifestation of fragile (X) mental retardation.

Classical infantile autism occurs more frequently in males and has recently been noted in patients with the fragile (X) form of X-linked mental retardation (XLMR). In order to better understand this association and to determine whether fra(X) XLMR could account for the excess of autistic males, we investigated a group of institutionalized severely handicapped adults, 33 males and eight females, who were diagnosed as autistic using the DSM III diagnostic criteria of infantile autism. Chromosome studies using FUdR showed that three of the males had the Xq27 fragile site. We confirmed the association of autism and fra(X) XLMR, and showed that this extreme form of behaviour is part of the spectrum seen in the Martin-Bell syndrome. Two of the three autistic males with the Xq27 fragile site had a history of birth insults, which in combination with developmental deficits due to the fragile X gene, might have led to the behavioural disorder. Even though the fragile X cannot account for the excess of males with classical autism, it is an important X-linked factor in its cause. The diagnosis can allow more accurate counselling for this subset of autistic males.