Evidence of a genetic basis for autism is strong. If one of a pair of identical twins is autistic, 90 percent of the time the other is, too, says psychiatrist Eric Hollander, director of the Seaver Center for Autism Research and Treatment at Mount Sinai School of Medicine. Researchers have found genetic mutations linked to autism, and most believe several such mutations must occur before autism results. In an April report in the American Journal of Psychiatry, Mount Sinai researchers reported the discovery of a gene that doubles the odds of autism in those who carry it, or one of its variants.

"We know there are genetic factors," Hollander says, but something else may also be involved. "It is possible there are environmental factors" that trigger the genes.

Several possibilities have been investigated, from junk-food diets, which are high in fatty acids that could interfere with the coating of nerve cells in the brain, to a drug used to induce labor in pregnant women. But no environmental link has been found.

Embryologist Patricia Rodier of the University of Rochester Medical Center and her colleagues say whatever happens to the developing brain may occur very early in pregnancy. "We're fascinated by the fact that symptoms of autism appear in a number of congenital conditions that we know arise a few weeks after conception," she says. In addition, maternal exposure to some substances known to cause birth defects, such as the German measles virus and alcohol, also increases the risk of autism.

Another area of interest is the possible link between autism and immune-system disorders.

An immune-system link is supported by some doctors and parents of autistic children who believe that a mercury-based preservative that has been used in childhood vaccines is at the core of the problem.

Thimerosal is being phased out of most vaccines for children, but some lots that contain the preservative are thought to be on doctors' shelves. Some studies have suggested that children with autism are less able than others to expel mercury from their systems, and some parent groups think multiple vaccines given to young children weaken the immune system and allow a build-up of mercury in developing brains.

Other scientists say this theory has been thoroughly investigated, but no conclusive evidence has been found. They say a continued focus by activists on thimerosal may undermine confidence in vaccines and divert funding from more promising theories. The Institute of Medicine, which provides advice on science and health to government, is expected to release a report today that conclusively dismisses the theory of thimerosal as a cause of autism.

Today, Mark Kelly is an affectionate, talkative boy. He loves his sister Laura, 10, and likes to play, though he doesn't play with other children, his mother says. He's in a special school for autistic children.

Progress is being made and there is more information available to parents than there was even five years ago, Susie Kelly says.

"We had false hopes he'd be cured," she says. "That didn't happen. He's clearly autistic. He's able to talk about what he needs or wants to do, but he can't really hold conversations. ... Now we're looking toward a future where I don't think he'll be able to live on his own. I don't think he'll ever have a driver's license or a girlfriend."