The present invention discloses the identification of a human autism susceptibility gene, which can be used for the diagnosis, prevention and treatment of autism and related disorders, as well as for the screening of therapeutically active drugs. The invention more specifically discloses that the SLC6A1 or SLC6A11 gene on chromosome 3 and certain alleles thereof are related to susceptibility to autism and represent novel targets for therapeutic intervention. The present invention relates to particular mutations in the SLC6A1 or SLC6A11 gene and expression products, as well as to diagnostic tools and kits based on these mutations. The invention can be used in the diagnosis of predisposition to, detection, prevention and/or treatment of Asperger syndrome, pervasive developmental disorder, childhood disintegrative disorder, mental retardation, anxiety, depression, attention deficit hyperactivity disorders, speech delay, epilepsy, metabolic disorder, immune disorder, bipolar disease and other psychiatric and neurological diseases including schizophrenia.