Because of SNPs, a person's genome may express a very active carcinogen-making protein, or a sluggish one, or something in between.

A protein with a very active binding site can "grab" more precarcinogen than usual. Or the protein may convert the precarcinogen to the carcinogen at a faster rate. In both cases, more carcinogen molecules pile up in the lungs, causing damage to the cells' DNA, which can lead to cancer.

On the other hand, if the SNPs result in a slow carcinogen-making protein, the lung is exposed to fewer DNA-damaging carcinogens, and the chance of cancer is reduced.

But until the body is exposed to cigarette smoke, no effect is seen.