Researchers have uncovered more information about how genetics can determine the risk of schizophrenia and manic depression.
The researchers discovered more about how the brain works
Glasgow and Edinburgh University experts found damage to a gene involved in how the brain thinks increases risk.
The team, writing in the magazine Science, say the finding could lead to the development of new drugs.
But the researchers stress environmental factors also influence the odds of developing mental illness.
Five years ago, the Scottish team identified a key faulty gene called DISC1, which acts as a "hub" gene, controlling other "spoke" genes around it.
In the latest work, the team have shown that a second gene - phosphodiesterase 4B (PDE4B) - also plays a crucial role.
This gene was already known to be involved in brain development and memory storage.
Damage to either or both genes can increase the risk of a mental health problem, the researchers say.
The extent of the flaws influence what kind of illness someone has, and its severity, they suggest.
Both genes determine the behaviour of one specific protein, which interact with each other.
It is this interaction which scientists think could be targeted with drugs, perhaps by modifying the amount or quality of the DISC1 protein, or by changing the way the PDE4B protein behaves.
The researchers worked along with scientists from the pharmaceutical company Merck, Sharp & Dohme Limited.
'Tantalising, but speculative'
Professor David Porteous at the University of Edinburgh, who led the work, said: "It is now clear that the DISC1 gene plays an important role in the risk of developing schizophrenia or bipolar affective disorder [manic depression].
"The new genetic link we have made to PDE4B and how that links back to DISC1 sheds much needed light on these debilitating disorders. It also suggests a new way of thinking about developing better and effective medicines."
But he added: "Risk isn't all controlled by genes. It's very much an interaction between genes and environmental factors."
But in a commentary in Science, Dr Akira Sawa and Dr Solomon Snyder of the Departments of Psychiatry and Neuroscience at Johns Hopkins University, warned: "However tantalising, the prediction of drug effects is still highly speculative."
Paul Corry, of the mental health charity Rethink, said: "What this research underlines is that schizophrenia is a very complex condition involving an interaction of various genes in ways that have not been defined.
"The research offers wider clues to the interaction between genes and any research that seeks to shed light on these complex interactions is to be welcomed, but the results should not be presented as the definitive cause of schizophrenia."
He added: "While we welcome any new research or progress into understanding the causes of schizophrenia, it would need to be checked before it would make a difference to the thousands of people living with severe mental illness in the UK."