A new genetic study shows even siblings with autism often have very different DNA mutations from one another a finding that strengthens the evidence that autism is often just genetic bad luck.

The deep dive into the DNA of 170 people with autism spectrum disorder shows that more than 69 percent of brothers and sisters with autism had different DNA mutations underlying their disorders, Dr. Stephen Scherer of The Hospital for Sick Children in Toronto, Canada and colleagues found.

"It's random mutation in these families. It just happens to be lightning striking twice," Scherer said.

"It just happens to be lightning striking twice."

The findings, published in the journal Nature Medicine, add to other research that shows "autism" is a broad term for a range of developmental disorders that are barely related to one another. The catchall name could be making it seem like one disorder is affecting many kids, when in fact it's a range of conditions.

"It is largely just a random effect," Scherer said. "Everybody in the population accumulates new mutations in the genome when they are conceived."

Autism spectrum disorder can range from the mild social awkwardness, including Asperger's syndrome, to profound mental retardation, debilitating repetitive behaviors and an inability to communicate. There's no cure, but experiments with early treatment suggest it can help.

Autism is becoming more and more common among U.S. kids, and researchers don't quite understand why. The last survey by the Centers for Disease Control and Prevention showed 2 percent of U.S. children have been diagnosed with an autism spectrum disorder one in 68 kids.

Studies also show it's clear that genetic mutations are responsible for a lot of it. Autism can run in families.

Scherer's team set out to see if siblings with autism were inheriting some common pattern of mutations from their parents. They recruited 85 families where more than one child had been diagnosed, and sequenced everyone's entire genome.

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Autism Genes Randomly Mutated, Study Finds