Many tumor mechanisms are known to circumvent immune cytotoxic responses, such as a decrease in human leukocyte antigens (HLA) and IFNγ expression; decreased neoantigen load in genetically stable tumors; an immunosuppressive TME rich in TGFβ/IL-10, IDO, or Treg/MDSC populations; decreased expression of T cell receptor coactivators; and increased expression of checkpoint inhibitors [11,90,91]. Here, IDO1 is linked to neoplasm.