A major new discovery may help scientists figure what causes autism, which is diagnosed in one of every 150 children, doctors say.

The answer might come from looking deep within the human genome, said geneticist Dr. David Miller, with Children's Hospital in Boston.

Although all humans are born with 23 pairs of chromosomes, scientists found a mutation in a small number of autistic children. In one particular chromosome, these children were either missing or had too much DNA, or deoxyribonucleic acid.

"It would be like a recipe where if you added twice as much salt or half as much salt, and it changes how the recipe turns out, because everything is in a delicate balance, especially inside the human brain," Miller.

How this mutation might lead to autism is still unknown. Scientists must still figure out the mutation's cause and if it is present in all or just a few autistic children.

In the near future, however, scientists hope to be able to use technology to diagnose children with autism at an early age. Pinpointing this mutation might be one clue in the diagnosis, Miller said.

"If it's a puzzle that has 100 pieces, this is one piece," Miller said.

Although a mercury-based preservative was widely believed to cause autism, a study released last week showed that the autism rate in children continued to rise after the preservative was removed in 2001.

The preservative has still been used in some flu shots, but the study adds to existing evidence that argues against linking vaccines to autism.

Peter and Elizabeth Bell, the parents of an autistic boy, said the discovery of the genetic mutation raises their hopes for their son's future.

"There's a palpable sense within the community that something significant is going to break," Peter Bell said. He works for Autism Speaks, a group that helped with the research that found the mutation.

"There is this search and this need to find an answer for what made this happen," Elizabeth Bell said.