The brains of people with autism are chemically different from those without autism, according to researchers.
A study, published in the journal Nature, showed the unique characters of the frontal and temporal lobes had disappeared.
Different genes should be active in each region, but autistic brains had the same pattern of gene expression.
The National Autistic Society said the results could be important for future treatments.
Autism spectrum disorders, including Asperger's syndrome, are common and affect more than 500,000 people in the UK.
They are thought to be caused by a combination of genetics and the environment.
Professor Daniel Geschwind, from the University of California, Los Angeles, said: "If you randomly pick 20 people with autism, the cause of each person's disease will be unique.
"Yet when we examined how genes and proteins interact in autistic people's brains, we saw well-defined shared patterns. This common thread could hold the key to pinpointing the disorder's origins."
The scientists in the UK, US and Canada compared samples from 19 autistic brains and 17 without.
They noticed that 209 genes linked to the way brain cells work and talk to each other were working at a lower level in autistic brains while 235 genes linked to immune and inflammatory responses were expressed more strongly.
The researchers said many of these genes had already been linked to the condition.
They also noted that there was no longer a difference in the genes expressed in the frontal and temporal lobes in the brain.
Professor Daniel Geschwind said: "Instead, the frontal lobe closely resembles the temporal lobe."
It is likely due to defective brain development, they argue.
Richard Mills, director of research at the National Autistic Society said: "We are beginning to better understand the differences between the brains of people with autism and those without.
"If replicated these findings are important for the development of interventions which may reduce the more disabling effects of autism.
"They also confirm the importance of research that can shed light on underlying mechanisms. It is critical that we continue our investment in high quality research consortia."