In recent years, genome-wide association studies (GWAS) identified several regions containing genes associated with smoking-related behaviors, especially a cluster of three nAChR subunit genes, CHRNA5, CHRNA3 and CHRNB4 (11-13). Despite the important functions of α4* nAChR in ND, no GWAS yielded a significant signal near CHRNA4, though several candidate gene studies reported significant associations of CHRNA4 and smoking behaviors (14-20).
