ABCB1 and major depressive disorder: Several single-nucleotide polymorphisms in the ABCB1 gene were associated with depression severity, response status or dosage adjustments in depressive disorders indicating an involvement of ABCB1 in depression.70, 71, 72, 73 Fluoxetine, the antidepressant we used for in vitro LCL phenotyping, is a rather weak substrate of ABCB1.74, 75 One may speculate that the absence of a correlation between fluoxetine-induced ABCB1 expression and clinical response could reflect the low ABCB1 substrate properties of this antidepressant drug.