IDO1 and neoplasm: IDO1 protects tumor cells from being cleared through three mechanisms, which are tryptophan depletion (Curti et al., 2007; Hill et al., 2007; Xie et al., 2008; Chang et al., 2011), products toxicity of tryptophan metabolites (Myint and Kim, 2014) and inducing regulatory T cell proliferation (Fallarino et al., 2006a,b; Colombo and Piconese, 2007; Kim et al., 2007).