The discovery of "missing" genes could help scientists understand how autism develops, a study suggests.
US researchers looked at the genetic profiles of more than 431 people with an autistic spectrum disorder (ASD) and 379 without.
They found those with an ASD were more likely to have just one copy of certain genes, when they should have had two.
UK experts said genetic factors were one promising area of research into the causes of autism.
About 1% of the population has an ASD. They can run in families - but scientists have not identified a cause.
Gene deletions or additions happen in everyone - it is why people are different.
It is which genes are affected that determines what the effect is.
There were far more gene deletions in the ASD group, and they were more likely to have multiple deletions.
Writing in the American Journal of Human Genetics, the team from Mount Sinai suggests this "mis-wiring" could alter the activity of nerve cells in the brain.
Prof Joseph Buxbaum, who led the research team, said: "This is the first finding that small deletions impacting one or two genes appear to be common in autism, and that these deletions contribute to risk of development of this disorder."
The researchers found many of the most common deletions in the autistic group were linked to autophagy - kind of waste-disposal and renewal process for cells,
Prof Buxbaum said: "There is a good reason to believe that autophagy is really important for brain development because the brain produces many more synapses [connections through which brain cells communicate] than it needs, and the excess needs to be pruned back."
He added: "Too many, or too few, synapses have the same effect of not making communication work very well. It could mean that some synaptic connections come in too late and may not solidify properly."
Difficulties communicating and interacting with others are common in people with an ASD.
The researchers suggest these genetic variations could be targets for genetic testing.
Carol Povey, director of the Centre for Autism at the UK's National Autistic Society said: "The causes of autism are still being investigated.
"Many experts believe that the pattern of behaviour from which autism is diagnosed may not result from a single cause.
"There is strong evidence to suggest that autism can be caused by a variety of physical factors, all of which affect brain development.
"Genetic factors may be responsible for some forms of autism, but it is likely to have multiple genes responsible rather than a single gene.
"This study provides interesting insights into autism and genetic factors.
"More research needs to be done before any concrete conclusions can be drawn about the causes and what this might mean for future developments, so it will be interesting to see how this research is built on to further enhance our knowledge of the condition."