These nAChR subunits are the principle targets of nicotine, and can also bind to two nicotine-specific metabolites namely: 4-(Methylnitrosamino)-1- (3-pyridyl)-1- butanone (NNK) and N-nitrosonornicotine (NNN) [12], [23], which are potent carcinogens and the effects of which on DNA are regarded as the primary cause of smoking-related cancers [24]. The gene discussed is CHRNA4; the disease is cancer.