Hypothesis: homozygosity in Tourette syndrome.

We review evidence suggesting that many individuals with Tourette syndrome (TS) may be homozygous for a "Tourette syndrome" gene. This is based on experience with pedigrees on 1,200 TS families, comparison of the occurrence of tics or associated behaviors such as obsessive-compulsive behavior, panic attacks, attention deficit hyperactivity disorder, and/or severe alcohol or drug abuse, on both the maternal and paternal side in 170 TS families compared to control families, biochemical studies of blood serotonin and tryptophan levels, and other evidence. These observations suggest the inheritance in TS may be best described as semi-dominant, semi-recessive. Some of the implications of this proposal are discussed.