Autistic disorder (AD) is a pervasive developmental disorder beginning before the age of 3 ys and lasting over the whole life time. It is characterized by a triad deficit: qualitative impairments in social interactions, communication deficits & repetitive and stereotyped pattern of behavior. Recent studies show prevalence rates ranging from 10 to 20 per 10,000. Although the etiology of AD is unknown, genetic predisposition is evident from family and twin studies, and heritability is estimated over 90%. The genetic component is best described by a multilocus model that takes into account epistatic interactions between several susceptibility genes. To date, suggestive autism susceptibility loci have been identified on almost every chromosome.