IDO1 and cancer: Silencing MUC1-C also decreased expression of (i) IDO1, which suppresses tryptophan (Trp) levels in the tumor microenvironment necessary for T-cell function (53), and (ii) tryptophanyl-tRNA synthetase (WARS, WRS) that protects cancer cells from Trp depletion (refs. 54, 55; Fig. 3G and H).