The incidence of autism appears to be as high as 1 in 88 children. The causes of autism remain unclear but a number of twin and familial studies have demonstrated strong support for the contribution of genetic changes in the risk of developing autism. There are some chromosome changes as well as known genetic conditions that can present with autism, including Fragile X and Rett syndromes. Recently it has been reported that the combined contribution of known genetic etiologies accounts for 40% of autism cases. There has been a long-standing association between large head size (macrocephaly) and autism. This has been confirmed by many studies. Cases of extreme macrocephaly have been correlated to mutations in the gene phosphatase and tensin homolog (PTEN). As a result, PTEN testing is now the standard of care for cases who present with autism and macrocephaly. A group of cases with autism and macrocephaly have been identified that do not have mutations in PTEN. Some cases of autism also present with overgrowth. This project will explore the genetic etiologies in these cases. This study seeks to identify the genetic changes in cases of autism with macrocephaly and thus expand the list of known genetic etiologies associated with this autism subgroup. Understanding the pathogenesis in these cases with autism will allow the possibility of early testing, treatment modalities and intervention therapies.