Hundreds of families were analysed
Tiny variations in a single gene might put some people at risk of developing type one diabetes, research suggests.
Subtle variations in the sequence of the CTLA-4 gene may also increase the risk of other types of autoimmune disease, such as Graves disease and autoimmune hypothyroidism.
Autoimmune disorders, which affect around 5% of the population, occur when the immune system turns on itself and attacks the body's normal cells rather than invading organisms, or infected or abnormal cells.
CTLA-4 has been described as the "master regulator" of the immune system.
When working properly, it ensures the immune system is kept in check and does not begin to attack the body's own tissues.
However, the latest research, based on genetic studies in hundreds of families, suggests that if the structure of the gene is altered, this break is not applied.
The work was carried out by the Juvenile Diabetes Research Foundation/Wellcome Trust Diabetes and Inflammation Laboratory in Cambridge, in collaboration with the University of Birmingham.
Damage to pancreatic cells causes diabetes
Researcher Professor John Todd said the research would not lead to immediate benefits for patients.
But he added: "If we obtain this sort of information about the many genes that are involved in autoimmune disease and if we keep building that jigsaw, way off in the future we will understand what is really causing the disease.
"If we do understand the causes it should be possible to modulate the primary reasons why some people develop this disease and others don't."
Type one diabetes affects 350,000 people in the UK and tends to set in during childhood and in the early 20s.
The body attacks its own cells in the pancreas which produce insulin.
People suffering form type one diabetes have to monitor their blood glucose levels and inject themselves daily with insulin to stay alive.
Both Graves disease and autoimmune hypothyroidism are disorders of the thyroid gland, which produces hormones that influence many of the body's functions.
Hypothyroidism is caused by the under-production of hormones. Symptoms may include tiredness, forgetfulness, brittle nails, dry skin, puffy face and eyes, depression and unexplained weight gain.
Grave's disease is the exact opposite. Over-production of thyroid hormones can lead to weight loss, nervousness, heart palpitations and irritability.
The research is published on the website of Nature magazine.