High IDO1 was not associated with OS in the entire cohort and in the immunotherapy-naive colorectal population as follows: OS from advanced/metastatic disease regardless of immunotherapy treatment (IDO high: n = 17, IDO intermediate/low: n = 116), HR = 0.94 (95% CI: 0.47–1.90, p = 0.87) and OS from advanced/metastatic disease in immunotherapy-naive patients (IDO high: n = 11, IDO intermediate/low: n = 57), HR = 1.43 (95% CI: 0.62–3.29, p = 0.4). Here, IDO1 is linked to metastatic neoplasm.