SLC6A4 and major depressive disorder: Previous studies have found several genes that may be associated with migraine combined with depression, including the promoter region of the 5-HT transporter (5HTTLPR) SLC6A4 gene (21), the dopamine receptor gene DRD2 and DRD4 (22), and the GABAergic system of the GABRQ and GABRA3genes (23).