IDO1 and neoplasm: In terms of tumor type, IDO1 expression was not associated with OS in nasopharyngeal carcinoma (n = 2, I2 = 85.6%, P = 0.922) but was related to worse OS in diffuse large B-cell lymphoma (n = 2, I2 = 0, P < 0.05) and hepatocellular carcinoma (n = 3, I2 = 0, P < 0.001).