CXCL8 and major depressive disorder: Although CXCL8 has not been proposed as a biomarker of MDD in previous gene expression studies, abnormalities in protein levels of pro-inflammatory cytokines, including IL-8, have been reported in MDD56–59, and alterations in the expression of genes related to cytokines other than IL-8 have also been observed in previous studies, e.g., alteration of CCL24 gene expression in peripheral blood from MDD patients was identified using targeted gene expression analysis60.