IDO1 and neoplasm: Overexpression and activation of IDO1 by tumor cells and immune cells reduces CD8+ effector T cells and accumulates Foxp3+ regulatory T cells in the tumor microenvironment by depleting essential amino acid Trp and generating toxic metabolites such as kynurenine (Kyn), which ultimately leads to tumor immune escape (Takikawa et al., 1986; Routy et al., 2016).