CHRNA6 and nicotine dependence: Besides the CHRNA4, CHRNB2 and CHRNA5/A3/B4 cluster on chromosome 15, which has been investigated intensively, recent evidence from both genome-wide association studies and candidate gene-based association studies has revealed the crucial roles of the CHRNB3–CHRNA6 gene cluster on chromosome 8 in nicotine dependence (ND).