As IDO1 function (resulting in tryptophan depletion and accumulation of metabolites) influences T-cell metabolism, we investigated whether the negative prognostic effect of IDO1 expression at the tumour invasion front in pT1-4N1Mx-staged CRC was associated with a decrease in the number of CD3+ or CD8+ T lymphocytes. The gene discussed is IDO1; the disease is colorectal carcinoma.