The correlations between serum neopterin and the serum kynurenine/tryptophan ratio in both colorectal cancer patients and in ‘no cancer’ controls, and the rise in the kynurenine/tryptophan ratio in colorectal cancer patients suggested an interferon-γ-induced increase in IDO activity (Murr et al, 2000) among colorectal cancer patients. The gene discussed is IDO1; the disease is cancer.