Further evidence in support of increased kynurenine pathway activation and subsequent tryptophan depletion in depression stems largely from the study of cytokine-induced depression, which occurs in 30% to 50% of medically ill patients being treated with IL-2 or IFN-α for cancer and Hep C (Musselman et al., 2001; Capuron et al., 2002; Capuron and Miller, 2004). This evidence concerns the gene IL2 and major depressive disorder.