SLC6A4 and major depressive disorder: Depression has been by far the most studied pathology (n = 102/315 papers, 32.3%), with studies focusing mainly on 5HTTLPR (n = 79/102, 77.5%) and BDNF (n = 20/102, 19.6%), and the remainder investigating 5HTR (1A/1B/2A/2C/3A) (n = 7/102, 6.9%), CRHR1 (n = 7/102, 6.9%), MAOA (n = 6/102, 5.9%), and OXTR (n = 6/102 papers, 5.9%).