Genetics may be to blame for hooking some people on cigarettes. Genetics may make some smokers more prone to lung cancer too, say three new studies. This is the strongest case so far for the biological foundation of nicotine addiction.

Scientists have pinpointed genetic variations related to smoking that could one day lead to screening tests and customized treatments for those trying to kick the habit.

The gene variations, which govern nicotine receptors on cells, could help explain some of the mysteries of chain smoking, nicotine addiction, and lung cancer -- like why a 90-year-old lifetime smoker never gets cancer, why some people can occasionally light up and never get hooked, and why some people have such a hard time quitting.

Initially, researchers are pretty certain that a smoker who inherits these genetic variations from both parents has an 80 percent greater chance of lung cancer than a smoker without the variants. That same smoker tends to light 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 U.S. and European governments and published Thursday in the journals Nature and Nature Genetics, looked at more than 35,000 white people of European descent in Europe, Canada, and the United States. Blacks and Asians will be studied soon.