PET imaging studies using newer radioligands for biomarkers of inflammation other than TSPO, such as monoamine oxidase B, cyclooxygenase, colony stimulating factor 1 receptor, and the purinergic P2X7 receptor may eventually confirm the state of neuroinflammation in MDD (Narayanaswami et al., 2018; Zhou et al., 2021). This evidence concerns the gene TSPO and major depressive disorder.