Scientists are hailing a major breakthrough in autism research, saying the finding has pinpointed its genetic causes.
The breakthrough could be the key to unravelling the condition and therefore identifying possible treatments.
Researchers are likening the finding to a similar breakthrough in the understanding of cancer 30 years ago and have called it a "monumental achievement".
Autism covers a group of conditions, marked by an impaired ability to communicate and interact socially and repetitive behaviour, which affect about one in 150 mostly male children.
Now, three major studies have for the first time identified the jigsaw pieces that cause the condition.
Dr Margaret Pericak-Vance, director of the Miami Institute for Human Genomics, who also took part in the US research, said: "Until now, no common genetic variant has been identified with such overwhelming evidence to support its role in autism spectrum disorders.
"The identification of a common variant for autism is a monumental achievement."
Dr Philip Johnson, chief scientific officer at The Children's Hospital of Philadelphia, said: "This comprehensive research opens the door to more focused investigations into the causes of autism disorders.
"It moves the field of autism research significantly ahead, similar to the way oncology research progressed a few decades ago with the discovery of specific genes that give rise to cancers."
Two of the new studies were American-led and reported in an early online edition of the journal Nature. The third was conducted by British scientists at Oxford University and appears in the journal Molecular Psychiatry.
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