Several family-based [20], [21], [22], [23], [24], [25], [26], [27], [28], [29], [30] and case-control [22], [24], [29], [30], [31], [32], [33] studies have investigated the association between specific polymorphisms within SLC6A2 and ADHD. This evidence concerns the gene SLC6A2 and attention deficit-hyperactivity disorder.