A rare genetic variation dramatically raises the risk of developing autism, a large study showed, opening new research targets for better understanding the disorder and for treating it.

Research into the causes of autism has focused on genetic causes because so many families have multiple children with the disorder. Thus far, only about 10 percent of autism cases have a known genetic cause. Boston-area researchers estimate the gene glitch they've identified accounts for another 1 percent of cases.

They found a segment of a chromosome which has genes linked to brain development and various developmental disorders was either missing or duplicated far more often in autistic people. The defect was inherited in some cases, but more often the result of a random genetic accident.

The results from the Autism Consortium study, released online Wednesday by the New England Journal of Medicine, confirm those of smaller studies by U.S. and Canadian research groups in the past year. The consortium verified its findings by checking two other DNA databases.

"They really did nail it," said Dr. Andrew Zimmerman, director of the Kennedy Krieger Institute's Center for Autism & Related Disorders in Baltimore, who was not involved in the research.

He predicted children newly diagnosed with autism or other developmental disorders now will be tested for this defect on chromosome 16 and that studies of many more DNA samples may reveal other autism-related gene variations.

Already, the findings are starting to be used to give some parents long-sought answers to burning questions: What caused autism in their child and how likely is it that any future children also would have autism, long known to run in families?

"We've provided very compelling evidence that this particular small stretch of the genome provides an important clue to the biological roots of autism," said lead researcher Mark J. Daly, an assistant genetics professor at Harvard Medical School and an investigator for the consortium, which includes researchers from 14 Boston-area universities and medical centers.

When the biological pathways involved are figured out, scientists can try to design drugs to target chemicals in the brain to treat autism, said Geraldine Dawson, chief science officer of the advocacy group Autism Speaks.

Autism, a complex, poorly understood disorder, is characterized by repetitive behaviors and poor social interaction and communication skills.