Families were ascertained for having one or more autistic children and at least one non-autistic child aged 16 or older for an extremely discordant sib-pair linkage study. Recruitment took place in Massachusetts and surrounding states through contacts with parent support and patient advocacy groups, brochures, newsletters, and the study web site. Parents were interviewed about their children, and non-autistic children were interviewed about themselves. An informant/caregiver, usually the proband’s mother, was interviewed using the Autism Diagnostic Instrument-Revised (ADI-R) to confirm the diagnosis of autism at age 4–5 years25,34. Families were included if the affected children met Diagnostic and Statistical Manual of Mental Disorders-IV (DSM-IV) criteria for autistic disorder and their non-autistic siblings (aged 16 and older) did not display any of the broader autism phenotype traits, which were assessed with the (M-PAS-R), the Pragmatic Language Scale (PLS), and the Friendship Interview 35,36. Probands were excluded if they had medical conditions associated with autism such as fragile X syndrome or gross CNS injury, or if they were under four years of age, due to the possible uncertainty in diagnosis at younger ages. Twenty-nine families met eligibility criteria for the study and comprised the final sample for analysis.
