Evidence from Mendelian randomization (MR) studies, which use genetic variants regulating levels/activity of a biomarker as proxies to address the issue of confounding, suggest that IL-6 and CRP could be potentially causally related to depression and psychosis (Hartwig et al., 2017; Khandaker et al., 2018c; Khandaker et al., 2020). Here, IL6 is linked to major depressive disorder.