These findings could help scientists understand autism at a molecular level and seek ways to prevent it, Dawson added.

The U.S. Centers for Disease Control and Prevention estimated in May that autism was very common among U.S. children -- with up to one in every 175 with the disorder. That would mean at least 300,000 U.S. schoolchildren have autism, a condition that causes trouble with learning, socializing and other behavior.

The researchers examined the DNA of 169 families with at least two siblings who met the strict criteria for autism.

They also scanned the DNA of 54 other families that had members with autism and less-severe forms of the disorder, such as Asperger syndrome.

They came up with "strong support" for an autism gene on chromosome 7 and "less, but still compelling evidence" for genes on chromosomes 3, 4 and 11, said Gerard Schellenberg, a professor of medicine at the University of Washington who led the study.

"It is highly unlikely that there is only one gene responsible for autism," Schellenberg said in a statement.

"There may be four to six major genes and 20 to 30 others that might contribute to autism to a lesser degree," he added.

"And because autism is rarer in females, it may take more risk genes for a female to have autism. There also is the possibility that there might be a biological difference in autism for females versus males."

The researchers said they were looking for genes that might put children at higher risk of autism so they could begin early treatment or perhaps even one day find a way to prevent the condition.

"Once we discover these susceptibility genes, we can immediately screen infants to identify those at risk early in life. Early identification can lead to early intervention, which could have a much more dramatic effect," Dawson said.

"Also, when a gene is discovered, you discover the underlying biology of autism at the molecular level. Once you understand the biology you can develop a prevention strategy including medical approaches," Dawson said.