DRD2 and nicotine dependence: This haplotype encompasses Neural Cell Adhesion Molecule 1 (NCAM1), a gene with a fundamental role in neural development, the Dopamine Receptor D2 (DRD2), a gene previously associated with ADHD, and the Tetratricopeptide Repeat Domain 12 (TTC12), and Ankyrin repeat and Kinase Domain Containing 1 (ANKK1), two genes associated with externalizing symptoms and nicotine dependence (David et al. 2010; Ducci et al. 2011; Gelernter et al. 2006; Yang et al. 2007, 2008) (Table 1).