The immunosuppressive IDO halts NK cell cytotoxicity, inhibits the infiltration and function of CD8+ T cells, recruits and activates MDSCs and Tregs in the TME, stimulates differentiation of CD4+ T cells into Treg phenotypes through activation of AhR by Kyn, induces tumor tolerance to apoptosis, impairs TCR via Vav1 elimination, elicits differentiation of immunogenic DCs into tolerogenic DCs, stimulates polarization of macrophages into TAMs, exhorts resistance to checkpoint inhibitors, and suppresses CD19-CART cells (228, 232–238). Here, IDO1 is linked to neoplasm.