Autism is a disorder of neural development characterized by impaired social interaction and verbal and non-verbal communication, and by restricted, repetitive or stereotyped behavior. The diagnostic criteria require that symptoms become apparent before a child is three years old. Autism affects information processing in the brain by altering how nerve cells and their synapses connect and organize; how this occurs is not well understood. It is one of three recognized disorders in the autism spectrum (ASDs. Researchers who compared the DNA of patients with autism and intellectual disability to that of their unaffected siblings found that the affected siblings had significantly more “runs of homozygosity,” or blocks of DNA that are the same from both parents. The finding suggests a role for recessive inheritance in this autism subgroup and highlights homozygosity as a new approach to understanding genetic mechanisms in autism.