SLC6A2 and attention deficit-hyperactivity disorder: Polymorphisms in the norepinephrine transporter gene (NET1, SLC6A2) also have been associated with ADHD (e.g., Brookes et al. 2006; Kim et al. 2008), although meta-analytic findings have not been strong for NET1. Overall, the associations from candidate gene studies have been very modest, with no gene accounting for > 3–4% of the total variance in ADHD phenotype (Smith et al. 2009).