IL18 and major depressive disorder: To do so, we designed a four-step study: (1) confirming IL-18 protein expression in human neurons, (2) comparing gene expression in IL-18-deficient mice with that of the mouse chronic mild stress (CMS) model of depression, (3) evaluating differences in gene expression between the PFC of Il18−/− and Il18+/+ mice by reverse transcription quantitative polymerase chain reaction (RT-qPCR), and (4) measuring the protein expression encoded by identified genes in serum, the PFC, hypothalamus, and hippocampus of Il18+/+ and Il18−/− mice.