Autism has a prevalence of ~0.6% in the general population and is four times more prevalent in boys than in girls. Together with four other disorders (Asperger’s disorder, childhood disintegrative disorder, Rett syndrome and Personality Disorder Not Otherwise Specified) it belongs to the group of Pervasive Developmental Disorders (PDD). Autism is the most common PDD and usually appears during the first three years of life. Its symptoms include impairments in verbal and nonverbal communication, lack of social interaction, and restricted and stereotypical behaviour [8]. Though autism is one of the most hereditary disorders in psychiatry, with an estimated heritability of up to 90% (Table 2) [9], the search for susceptibility genes has proven to be complex. Until now, a number of chromosomal loci have been identified that may represent regions predisposing to autism, including regions on chromosome 1p12-p21.1, 1q21-q44, 2q24.1-q33.1, 3q21.3-q29, 4q21.3-q35.1, 5p12-p15.33, 6q14.3-q23.2, 7q21.2q36.2, 10p12.1-p14, 10q23.3-q26.3, 13q12.13-q33.1, 15q13.1-q26.1, 16p12.1-p13.3, 17q11.1-q21.2, 19p13.11-p13.3 and 19q12-q13.12 [reviewed in 10]. Although most susceptibility regions have been studied in more detail via the candidate-gene approach (e.g. the Reelin gene on chromosome 7q22 and the
