A related hypothesis focuses on age-dependent microglial dysregulation, whereby CNS immune changes in the elderly brain lead to altered immune vigilance with microglia responding excessively to cytokine challenges, leading to IDO-1 upregulation and induced KYN formation; this microglial dysregulation is associated with increased depression and suicidality [54,55]. The gene discussed is IDO1; the disease is depressive symptom measurement.