Hundreds of spontaneous gene mutations may be responsible for the development of autism in families without a history of the disorder.

In three studies published Thursday in the journal Neuron, researchers from Yale University, Columbia University and Cold Spring Harbor Laboratory in New York looked at gene analyses from over 1,000 families where one child had an autism diagnosis but the parents and the child’s siblings did not.

They found that children with autism had more copy number variants, or sections of DNA that are duplicated or missing, as compared to their siblings without the disorder. The gene variants were wide-ranging, but were involved in the formation of synapses which are responsible for communication in the brain and other parts of the nervous system.

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“The diversity implies that a treatment for one form of autism may have no value for the majority,” says Dr. Michael Wigler of the Cold Spring Harbor Laboratory who led one of the studies. “This type of knowledge is a critical first step in developing novel treatment approaches.”

In addition, the researchers found that a greater number of gene variants seemed to be required in order to trigger autism in girls, which could help explain why the developmental disorder occurs more frequently in boys than girls.

One group of genes proved to be of particular interest to researchers because their addition or deletion resulted in two opposite phenomenon. When extra copies of this group were present, the genes triggered autism. But when this group was missing, Williams syndrome occurred, a condition characterized by extreme sociability and friendliness.

“This relatively small genomic interval clearly holds important clues to understanding the social brain,” said Dr. Matthew W. State from Yale University who led one of the studies.