A genetic link to the symptoms of schizophrenia has been found, according to researchers.
An Edinburgh University team found people carrying a variant of a gene called neuregulin had a higher chance of developing psychotic symptoms.
The findings, published in the journal Nature Neuroscience, could possibly point to new treatments.
The study followed 200 young people, all at a high risk of developing schizophrenia, for 10 years.
"You have to understand more about what causes diseases before you can start designing treatments for it"
Schizophrenia is known to run in families, and all of the volunteers had two or more relatives with the condition. And being aged between 16 and 25 at the start of the study, they were on the cusp of the period when symptoms were most likely to develop.
To investigate why some people go on to develop the condition and why others do not, the researchers carried out interviews, brain scans, psychological tests and genetic analysis.
They discovered participants who carried a variation of the neuregulin gene were much more likely to develop psychotic symptoms associated with schizophrenia, such as paranoia or hearing voices, than those without the gene variant.
Brain scans also revealed those carrying the variant gene were more likely to show abnormal brain activity in the frontal and temporal regions - areas often associated with schizophrenia.
Other studies have found the gene variant is involved with switching on and off a gene associated with brain development.
Designing treatments
Dr Jeremy Hall, lead researcher on the paper, based at the division of psychiatry, Edinburgh University, said: "These major mental illnesses have really been for a very long time a big black box in terms of what is causing them - it is not that long ago that people thought you got schizophrenia because you had a bad mother. And treatments have not advanced a lot over the last 50 years.
"You have to understand more about what causes diseases before you can start designing treatments for it.
"These results help us to understand how a gene might alter brain function and then cause symptoms, and could represent a target for treatments in the future."
Dr James MacCabe of the Institute of Psychiatry, Kings College London, said: "It is a very interesting study, and, potentially, quite an exciting one.
"We have known for years that schizophrenia had a genetic contribution, but until quite recently no genes had been found.
"Over the past five years there have been about 10 possible genes identified, and neuregulin is a very promising gene."
He said it was a relatively small study and would have to be repeated on a much larger scale, but if the results held true the findings would be significant.
"The next stage would be to understand how a defect in this gene causes schizophrenia so as to understand the neurobiology of the disorder and design treatments that could reduce the symptoms."
Marjorie Wallace, chief executive of the mental health charity Sane, said: "There have been so many hopes of finding a gene for schizophrenia which have ended in a cul-de-sac.
"If this latest research were to prove a breakthrough and lead to understanding what causes schizophrenia, we could at last find more effective treatments and potential cures, transforming the future for the one in 100 who suffer from this devastating condition."
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