In addition, Arredondo et al. 40 found that α3, α4, α7, α9, α10, β2 and β4 nAChR subunits can form the high-affinity sites to bind 4-(methylnitro-samino)-1-(3-pyridyl)-1-butanone (NNK), a cancerogen produced through nicotine nitrosylation, thus to increase the risk of lung cancer. This evidence concerns the gene CHRNA4 and lung carcinoma.