However, these data are influenced by cytokine immunotherapy, such as interferon (IFN)-alpha, that could in turn frequently cause depression (Schaefer et al. 2012), as discussed in the later section on “Implication for intervention.” Alterations of the kynurenine pathway could also mediate sleep disturbance, which is associated with elevated QUIN/KYNA ratio and increased C-reactive protein (CRP), as showed by Cho and colleagues in their interesting clinical study on depressed patients (Cho et al. 2017). Here, CRP is linked to major depressive disorder.