CHRNA4 and attention deficit-hyperactivity disorder: Comings et al. found a dinucleotide repeat in intron 1 as a risk for ADHD, Todd et al. identified polymorphism in the exon 2–intron 2 junction as a risk for severe inattention problems, Brookes et al. found the association between the 5’ flanking regions of CHRNA4 with combined subtype of ADHD; nevertheless, Kent et al. and Bobb et al. found no significant evidence of association in their studies (Kent et al., 2001b; Todd et al., 2003; Bobb et al., 2005; Brookes et al., 2006; Lee et al., 2008; Wallis et al., 2009).