IDO1 and major depressive disorder: Tsao et al. have reported that elevated levels of IFN‐γ may be associated with major depressive disorder (MDD) and that the enzyme “indoleamine 2,3‐dioxygenase (IDO)” plays a role in a depression‐related signaling cascade (Gabbay et al., 2009; Simon et al., 2008; Tsao, Lin, Chen, Bai, & Wu, 2006).