CHRNB4 and chronic obstructive pulmonary disease: Single nucleotide polymorphisms (SNPs) of the chromosome 15q25 region, which contains α5-α3-β4 nAChR gene cluster (CHRNA5-CHRNA3-CHRNB4), is frequently associated with nicotine- (tobacco-) dependence, chronic obstructive pulmonary disease (COPD), and lung cancer in genome-wide association studies (GWAS) [18].