A 'double fault' in one gene could lead to severe obsessive compulsive disorder, scientists suggest.
Many people with OCD wash their hands compulsively
People with OCD tend to repeat certain behaviours such as handwashing or cleaning.
Researchers from the US National Institute of Mental Health carried out the study which they say could lead to more treatments for the illness.
It is believed that OCD is linked to low levels of serotonin in the brain and that it may run in families.
DNA from 170 people, including 30 each with OCD, eating disorders and seasonal affective disorder plus 80 healthy people, was analysed by scientists.
They were looking for variations in the human serotonin transporter gene (hSERT) which controls the movement of the chemical between nerve cells in the brain.
Six out of seven of the people in two separate families who had one gene mutation had OCD.
Some also had anorexia nervosa, Asperger's syndrome, social phobia, tic disorder, and abused alcohol or other substances.
The four people who had the most severe symptoms also had a second mutation in the same gene.
Dr Dennis Murphy, who led the research: "In all of molecular medicine, there are few known instances where two variants within one gene have been found to alter the expression and regulation of the gene in a way that appears associated with symptoms of a disorder.
"This step forward gives us a glimpse of the complications ahead in studying the genetic complexity of neuropsychiatric disorders."
Dr Thomas Insel, director of the National Institute of Mental Health, said: "This is a new model for neuropsychiatric genetics, the concept of two or maybe more within-gene modifications being important in each affected individual."
Some people with OCD are treated with drugs that reduce the binding of serotonin to transporters such as selective serotonin reuptake inhibitors, (SSRIs), but the researchers say people with the hSERT gene defect do not seem to respond to them.
Dr Naomi Fineberg, a consultant psychiatrist who specialises in treating OCD at Queen Elizabeth II Hospital in Welwyn Garden City, told BBC News Online: "This research is potentially extremely exciting, although it needs to be replicated in larger studies.
"It focuses attention on genetic causes of OCD, and we think it is a genetic illness."
But she said much more research was needed to determine if these genetic faults were directly linked to the condition - or were just coincidence.
The research was published in the journal Molecular Psychiatry.