SLC6A4 and major depressive disorder: Expression of 5-HTT is directly correlated with 5-HT transporter function, which has been implicated as a susceptibility gene for several psychiatric disorders (e.g., affective disorders, schizophrenia, anxiety, autism, depression, suicide, OCD, and addiction [4]) as well as a pharmacogenetic determinant of interindividual SSRI response variability [5,6,7,8].