Scientists have pinpointed a genetic link that makes people more prone to get hooked on tobacco, smoke more cigarettes longer, and develop deadly lung cancer. The discovery by three separate teams of scientists makes the strongest case so far for the biological underpinnings of the addiction of smoking and how genetics and cigarettes combine in cancer, experts said. And it may lay the groundwork for more tailored quit-smoking treatments. "This is kind of a double whammy gene,' said Christopher Amos, a professor of epidemiology at the M D Anderson Cancer Center in Houston and author of one of the studies. "It also makes you more likely to be dependent on smoking and less likely to quit smoking.' A smoker who inherits this genetic variation from both parents has an 80 per cent greater chance of lung cancer than a smoker without the variants, the researchers reported. And that same smoker on average lights up two extra cigarettes a day and has a much harder time quitting than smokers who don't have these genetic differences. The three studies, funded by governments in the US and Europe, will be published on Thursday in the journals Nature and Nature Genetics. The scientists surveyed genetic markers in more than 35,000 people in Europe, Canada and the United States, zeroing in on the same set of genetic differences. They are not quite sure if what they found is a set of variations on one gene or on three closely connected genes. But they said the result is the same.