This level of explanation is comparable to the finding of three genome-wide association (GWA) studies which reported variation in 15q24-25, containing three nAChR genes (CHRNA5, CHRNA3, CHRNB4), contributing to lung cancer risk and associating strongly with amount of smoking and ND [52]–[54] and where less than 1% of the variance in number of daily cigarettes smoked was explained by alleles of these genes. Here, CHRNA4 is linked to lung cancer.