SLC6A2 and attention deficit-hyperactivity disorder: [21], [23], [26], [29], [30] A number of studies have examined the association between a functional SNP in the promoter region of the gene [−3081(A/T), rs28386840] and ADHD. [24], [28], [31], [32], [33] Furthermore, association between SLC6A2 and ADHD endophenotypes, including neurocognitive measures [34], [35] or quantitative symptom scores, [36] has also been studied.