IFNG and neoplasm: In addition, it has been shown that T cell-derived IFN-γ can interact with tumor cells and tumor-associated APC to induce the expression of indoleamine 2,3-dioxygenase (IDO), an enzyme that degrades the essential amino acid tryptophan that leads to the suppression of T cell immunity (Carlin et al., 1987; Munn et al., 2002; Fallarino et al., 2003; Zou, 2005; Sharma et al., 2007; Mellor and Munn, 2008; Muller et al., 2008).