Researchers at the Icelandic biotechnology company DeCode Genetics Incorporated said they found a gene that can double the risk of type 2 diabetes, a disease that affects at least 171 million people worldwide.

DeCode scientists reported yesterday that people who inherit two copies of the gene – one from each parent – are more than twice as likely to develop diabetes as those who don’t carry any copies. Being born with one copy of the gene raises the risk by about 45%, reported the research team led by StruanGrant.

The gene discovery, made by sifting through DNA samples from people in Iceland, may lead to a blood test to help predict who is at risk of developing diabetes sometime in their life.

Those identified at risk may reduce their chances of getting the disease with exercise and weight control, the chief executive officer of DeCode, Kari Stefansson, said.

“It is terribly important to know if you have this gene variant,” he said on January 11 by telephone. “It gives you an added incentive to exercise and eat right.”

Type 2 diabetes, which can develop silently in obese, inactive adults, occurs when patients’ bodies lose control over sugar levels in the blood. It often causes heart, nerve, eye, and kidney disease before patients know they have the illness.

Almost 21 million Americans have diabetes, and most suffer from the type 2 illness that normally occurs in adults, according to the American Diabetes Association. The DeCode diabetes gene may be responsible for about 21% of all type 2 cases in America, Mr. Stefansson said.

DeCode uses DNA analysis from the Icelandic population to discover disease causing genes. In its latest finding, the company’s researchers found that diabetes was more common in people born with a version of a gene called TCF7L2. The chemical units that compose genes often vary in slight ways from person to person, and certain variants have been found to make people more susceptible to diseases than others.

Researchers had not thought the gene, which may have an impact on the release of sugar from the liver, would play a key diabetes role, a genetics researcher at the Joslin Diabetes Center and Harvard Medical School, Alessandro Doria, said.

“It’s not completely out of left field, but it was not at the top of the list of candidates,” he said January 11 by telephone.

Diabetes is the leading cause of blindness and kidney failure in America. Other complications, such as heart disease, can also be serious, and the disease costs about $132 billion in treatment, lost work, and other spending in 2002.

Available diabetes drugs boost insulin production, slow the liver’s release of sugar, or increase sensitivity to insulin.

The relatively large impact of De-Code’s gene suggests its discovery may allow drugmakers to develop targeted medicines, just as some new cancer drugs act on genes involved in cancer, the president of the American Diabetes Association, Robert Rizza, said.

“If you do know how this gene is working, you might want to treat people have it with a drug that works on that pathway,” he said on Friday by telephone.

Type 2 diabetes occurs when the body stops responding to insulin, the hormone that lowers sugar levels in the bloodstream. Researchers have long said that along with lifestyle factors, such as being overweight and couch-bound, genes would also prove to raise diabetes risk.

Other genes implicated in the disease have only a weak impact on risk, Mr. Rizza said. After years without finding a gene that makes diabetes significantly more likely to strike, scientists began to doubt one existed, Mr. Doria said.

DeCode took its cue from earlier research linking diabetes in Icelanders and Mexican-Americans to a region on chromosome 10, Mr. Stefansson said. DeCode, which has detailed genetic data on about 100,000 Icelandic adults, studied 228 markers spread across that genetic region. The study analyzed 1,185 people with diabetes and 931 without.

After finding the association with diabetes in Icelandics, DeCode found the same TCF7L2 link in Denmark and American populations, Mr. Stefansson said. About 38% of Americans have one copy of the gene variant, and 7% have two copies, he said.

More studies will be needed to confirm the results and the importance of the gene in raising diabetes risk, Mr. Doria said.

The study appeared online yesterday in the journal Nature Genetics.