Researchers have identified a key gene that plays a role in the autoimmune disease Lupus.
The culprit is a defective version of a gene called OX40L that is important to several different types of immune system cell.
It affects lymphocytes, white blood cells which are one of the body's main defenders against infection.
Lupus, or Systemic Lupus Erythematosus (SLE), afflicts around 50,000 people in the UK, mostly women.
It causes a skin rash, joint pains and fatigue, and can lead to serious inflammation of the kidneys and other internal organs. People with Lupus are five times more likely to die early than members of the general population.
The gene variant is carried by one in six people in the UK. Having one copy of the Lupus gene raises the risk of developing the disease by 50%.
Professor Tim Vyse, from Imperial College London, who led the research funded by the Wellcome Trust, said: "Although it appears to have a number of environmental triggers, we are now beginning to get a clearer picture of the role that genetics also plays in the disease.
"Identifying a specific gene will enable us to see at a molecular level what is behind Lupus and develop treatments targeted at inhibiting or blocking the action of those genes."
The findings are published in the journal Nature Genetics.
Prof Vyse's group and other scientists have previously identified two other genes implicated in Lupus, IRFS and FCGR3B.