For instance, individuals diagnosed with major depressive disorder (MDD) show increases in circulating leukocytes and proinflammatory cytokine production [reviewed in Hodes et al. (2015)], with higher childhood levels of IL-6 and C-reactive protein (CRP) potentially predating the onset of depression (Khandaker et al., 2014). This evidence concerns the gene IL6 and depressive symptom measurement.