This paper introduces one of our projects performed at Hokkaido University. During the course of pharmacokinetic studies of SM-12502, which was under development as an anti-platelet-activating factor agent, we found three individuals who showed a slow metabolic phenotype in its pharmacokinetics. Analyzing the genes for CYP2A6 from the three, we discovered that they had the whole CYP2A6 gene deletion (CYP2A6*4C). Genetically engineered Salmonella YG7108 cells expressing human P450 were established to compare the mutagen-producing capacity of the P450 enzymes for various N-nitrosamines. We found that CYP2A6 was involved in the metabolic activation of N-nitrosamines with relatively bulky alkyl chains such as a tobacco-specific nitrosamine, 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone (NNK), which has been known to cause lung tumors in rodents. Thus, to examine the hypothesis that individuals possessing the CYP2A6*4C have a reduced risk of cancer due to the lack of the metabolic activation of certain carcinogens in tobacco smoke, a case-control study was performed. The results clearly indicated a significant association between the CYP2A6 genotype and lung cancer risk in smokers. In contrast, there was no significant relationship between them in nonsmokers. In addition, our results showed that the reduced risk of cancer was caused by the reduced activity of CYP2A6. Thus it was expected that the inhibition of the enzyme would result in a reduced cancer risk caused by smoking. The results of experiments using mice which were treated with NNK, a carcinogenic nitrosamine contained in tobacco smoke, together with 8-methoxypsolaren, a strong inhibitor of CYP2A6, indicated that the inhibition of CYP2A6 completely abolished the occurrence of adenoma.