In the clinical field, a meta-analysis of studies into major depressive disorder (MDD) concluded that two pro-inflammatory cytokines, interleukin (IL)-6 and tumour necrosis factor-alpha (TNFα), were consistently elevated in depression, while other cytokines, such as IL-1β, IL-4, IL-2, IL-8, IL-10 and interferon-gamma (INFγ), were not [7]. This evidence concerns the gene IL4 and major depressive disorder.