IDO1 and neoplasm: Studies have shown that IDO is an immunomodulator that inhibits T lymphocyte activity and produces immune tolerance to tumor cells.27 Several studies indicate that IDO aids cancer cells in escaping from the immune system during tumor growth and metastasis.28, 29, 30 Additionally, it has been proven that high expression of IDO in APCs inhibits APC activity and reduces antigen‐presenting efficacy.31 Therefore, regulation of IDO functions has great significance in regulating immune response to tumors.