CHRNA4 and nicotine dependence: Genome‐wide association study (GWAS) meta‐analyses have robustly reported that the CHRNA5‐CHRNA3‐CHRNB4 nAChR gene cluster on 15q25 and the CHRNB3‐CHRNA6 region on 8p11.21 are associated with smoking quantity (measured by cigarettes per day, CPD) and nicotine dependence (ND) (measured by the Fagerström Test for Nicotine Dependence, FTND (Heatherton et al.