CHRNB4 and nicotine dependence: Genome-wide association studies have identified single nucleotide polymorphisms (SNPs) as risk factors for nicotine dependence and lung cancer in genes encoding nAChR subunits (CHRNB3-CHRNA6 and CHRNB4-CHRNA3-CHRNA5 clusters), and in nicotine-metabolizing enzymes (CYP2A6 and CYP2B6) (Bierut et al., 2007; Kumasaka et al., 2012; Saccone et al., 2009; Thorgeirsson et al., 2010).