A gene mutation which affects brain development increases the risk of autism, scientists have suggested.
The researchers have found a tiny difference in the brains of people with autism
US researchers looked at 1,200 children with the condition.
Mutations were more common in children with autism and having the altered gene increased the risk of autism by more than double.
Experts said the findings, in Proceedings of the National Academy of Sciences, were interesting, but needed to be reproduced in other studies.
Autism is a developmental disability that affects the way a person communicates and interacts with other people.
The MET gene is known to be involved in brain development, regulation of the immune system, and repair of the gastrointestinal system.
All of these parts of the body can be affected in children with autism.
The researchers, from the Vanderbilt Kennedy Center for Research on Human Development, found that the mutation did not stop the gene working, but made it less active.
The mutation was common in children with autism, and appeared more frequently in families that had more than one child with autism.
Overall, this mutation raised the risk of autism by 2.27 times.
Writing in PNAS, the researchers said: "Although yet to be identified environmental factors likely contribute to the development of autism, heritability studies suggest that the impact of those factors must be imposed upon individuals genetically predisposed to the disorder.
Given the MET gene's known involvement in the development of the higher brain regions, the researchers say their findings could provide leads in pursuing the brain abnormalities that cause autism.
These results may also lead to improved understanding of medical conditions associated with the condition.
Professor Simon Baron Cohen, of the Autism Research Centre in Cambridge, said: "This is a thorough study documenting a potentially important association between autism and a common gene variant.
"Crucially, such studies depend on independent replication by another lab in another sample, to test for reliability of the association.
"If confirmed, it may help in understanding the underlying neurobiology of autism."