DRD4 and attention deficit-hyperactivity disorder: These variants implicated six genes: the serotonin transporter gene (5HTT), the dopamine transporter gene (DAT1), the D4 dopamine receptor gene (DRD4), the D5 dopamine receptor gene (DRD5), the serotonin 1B receptor gene (HTR1B) and a gene coding for a synaptic vesicle regulating protein known as SNAP25. A meta-analysis covering all genetic association studies of adults with ADHD reported a significant association between adult ADHD and BAIAP2 (brain-specific angiogenesis inhibitor 1-associated protein 2).