In MDD patients, we saw an ∼18% relative increase in TSPO expression, which was present in all investigated brain regions, and with effect sizes of comparable magnitude to those in previous PET studies of availability of serotonin transporters and monoamine oxidase A binding in MDD (Meyer et al., 2006; Gryglewski et al., 2014). This evidence concerns the gene MAOA and major depressive disorder.