Scientists are closing in on the genetic cause of autism, with the latest research pinpointing mutations in DNA inherited from the mother.
A team at Cornell University analysed DNA belonging to 903 children on the autism spectrum, and compared it with DNA from their unaffected siblings.
Children with autism had twice the number of potentially harmful mutations in their mitochondrial DNA than their siblings.
"The risk associated with these mutations is most pronounced in children with lower IQ and poor social behaviour," lead researcher Zhenglong Gu wrote in journal PLOS.
Mitochondria is the part of the cell which converts food into energy. Its DNA only comes from the mother, unlike nuclear DNA which is a mix of DNA from both the mother and father.
Mitochondrial DNA mutates about 10 times faster than nuclear DNA.
"Because mitochondria play a central role in metabolism, these findings may help explain the metabolic disorders commonly associated with autism spectrum disorder and other neurodevelopmental disorders," says Dr Gu, including epilepsy and problems with their organs, such as their liver or kidneys.
Research from Harvard Medical School released earlier this year showed a link between autism and genetic defects in the body's nervous system.
Another study found a link between the disorder and the lack of an antibody called immunoglobulin G type 1.
"The result of our study synergises with recent work on autism spectrum disorder," says Dr Gu.
Not all mitochondrial mutations are inherited from the mother though - they can happen spontaneously as the child grows up, thanks to the higher mutation rate.
Dr Gu hopes the findings will lead to new treatments for autism that focus on mitochondrial DNA.