CYP2A6 and nicotine dependence: Further evidence on the association of cyp2a6 with the number of cigarettes smoked per day and nicotine dependence is observed in the synergic effects of the chrna5/chrna3/chrnb4 cluster and this gene, showing independent and additive effects of allelic risk for these two chromosomal regions in two phenotypes (Wassenaar et al., 2011).