Chromosome findings in twins with early-onset autistic disorder.

In a twin study of autistic disorder, chromosome analyses were carried out in nine pairs of monozygotic (MZ) twins, two pairs of dizygotic (DZ) twins, one set of MZ triplets, one single twin from a MZ pair, and seven single twins from DZ pairs. All but one of the MZ sets were concordant for autistic disorder; all DZ pairs were discordant. Fragile X(q)(27.3) was found in one pair of MZ twins and in MZ triplets, i.e., in 9% of the population with autistic disorder. A marker chromosome of unknown origin was detected in a male twin with autistic disorder from a discordant DZ pair.