Washington ? Families claiming that a mercury-based preservative in vaccines triggers autism will challenge mainstream medicine today as they take their case to a federal court.

They seek vindication and financial redress from a government fund that helps people injured by shots.

Two 10-year-old boys from Portland, Ore., will serve as test cases that determine whether the children and their families in similar situations should be compensated.

Attorneys for the boys will attempt to show the boys were happy, healthy and developing normally. But, after being exposed to vaccines with thimerosal, they began to regress and show symptoms of autism.

Thimerosal has been removed in recent years from standard childhood vaccines, except flu vaccines that are not packaged in single-doses.

In 2004, a committee with the Institute of Medicine concluded there was no credible evidence that vaccines containing thimerosal caused autism.

Overall, nearly 4,900 families have filed claims with the U.S. Court of Claims alleging that vaccines caused autism and other neurological problems in their children. Lawyers for the families will present three different theories of how vaccines caused autism.

The Office of Special Masters of the claims court has instructed the plaintiffs to designate three test cases for each of the three theories – nine cases in all – and has assigned three special masters to handle the cases. Three cases in the first category were heard last year, but no decisions have been reached.

The two cases beginning today are among the three that focus on the second theory of causation: that thimerosal-containing vaccines alone cause autism.

The plaintiff in the third case originally scheduled for hearing this month has withdrawn and lawyers and court officials are working to agree on substitute case.

Hearings in the test cases for the third theory of causation are scheduled in mid-September.

Lawyers for the petitioning families in the cases being heard this month say they will present evidence that injections with thimerosal deposit a form of mercury in the brain. That mercury excites certain brain cells that stay chronically activated trying to get rid of the intrusion.

“In some kids, there’s enough of it that it sets off this chronic neuroinflammatory pattern that can lead to regressive autism,” said attorney Mike Williams.

Autism is a developmental disability that typically appears during the first three years of life and affects a person’s ability to communicate and interact with others. Dr. Andrew Gerber, a psychiatrist, said that medical experts don’t have a comprehensive understanding of what causes autism, but they do know there is a strong hereditary component.

Toxins from the environment could play a role, but currently, data does not support that they do, Gerber said.

Arguments are scheduled to go on throughout the month. A final decision could take several more months.