In order to limit the availability of tryptophan for cancer cells and to enhance T cell immune responses, we tested four inhibitors in vitro as follows: (i) 1-methyl-l-tryptophan (1-l-MT, an IDO-1 inhibitor); (ii) 1-methyl-d-tryptophan (1-d-MT, an IDO-2 and TDO inhibitor); (iii) tenatoprazole (a proton pump inhibitor and IDO-2 inhibitor); (iv) 680C91 (a TDO inhibitor) [50,52,59,60]. Here, IDO2 is linked to cancer.