Importantly, tryptophan is an essential amino acid of significant interest in cancer research and immunotherapy for several reasons: the high expression of tryptophan-degrading enzymes, such as Trp 2,3-dioxygenase (TDO) and indoleamine 2,3-dioxygenase (IDO), in cancer cells; the production of the immunomodulatory kynurenine metabolite resulting from tryptophan degradation; and the regulation of tryptophan levels by tumors to suppress immune defenses, thereby facilitating immune escape [34]. The gene discussed is IDO1; the disease is cancer.