— Smokers who don't have the variants are still more than 10 times more likely to get lung cancer than nonsmokers. Smokers without the variant have about a 14 percent risk of getting lung cancer. The risk of lung cancer for people who have never smoked is less than 1 percent, said another study author, Paul Brennan of the International Agency for Research on Cancer in Lyon, France.

Brennan and Amos, working on different teams, linked the genetic variation itself — when triggered by smoking — directly to lung cancer. Brennan said the nicotine receptors that the variants act on also can stimulate tumor growth.

But Stefansson said the increased lung cancer risk was indirect — the variants led to more smoking, which led to more cancer.

For Stefansson, the research hits home. His father, a smoker, died of lung cancer. And Stefansson, who doesn't smoke, frequently lectures his 23-year-old daughter "who smokes like a chimney." She acts as if she is immortal and smoking can't kill her, Stefansson said. But his own research shows that her genes are probably stacked against her.

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On the Net: Nature: www.nature.com/nature