The origin of the brain development disorder known as autism is unclear. A new study published this week in the New England Journal of Medicine provides another piece of the puzzle. The study points to a chromosome abnormality that appears to increase the risk of autism.

As many as one and a half million Americans have some form of autism, a mysterious brain disorder that impairs social interactions and communications -- in most cases starting before a child is 3 years old.

Researchers now have a new lead in predicting some cases of autism. The study found a chromosomal abnormality, specifically chromosome 16, is either missing or duplicated in about one percent of people with autism.

"This could be valuable as an identifying tool for children at risk at an earlier age when early intervention could be valuable," said Dr. Mark Daly of Massachusetts General Hospital, the lead author of the study.

Researchers say their genetic findings can help families better understand the risk to additional siblings, and provide new hope for treatments.

"A solid lead like this provides a solid biological target for therapeutics and drug discovery, and this is something we really didn't have for autism before this discovery," said co-author Dr. Rudolph Tanzi of Massachusetts General.

They're unclear why the abnormality increases the risk. They say this is only one important piece of a larger puzzle.

In a separate study released this week, the California Public Health Department found autism cases continued to climb even after a vaccine preservative, which some blame for causing autism, was removed from the childhood shots.

There's no cure for autism but greater understanding and early therapy are crucial for lessening the severity.

Some advocacy groups have blamed vaccines for autism. Nearly 5,000 claims alleging a vaccine link with the disorder have been lodged with the federal government, which is deciding whether victims should receive compensation from a government fund.