On the other hand, it may be that the exact regulatory mechanism of IDO1 in physiology and pathology and its impact on the tumor microenvironment are not well understood [74], and tumor cells or immune cells in the tumor microenvironment may express other tryptophan-degrading enzymes beyond IDOs (i.e., TDO), thus still escaping from immune surveillance in the case of IDO inhibition [75,76]. This evidence concerns the gene IDO1 and neoplasm.