Indoleamine-2,3-dioxygenase (IDO) expression has a controversial impact in the TME, with higher IDO levels correlated with an inhibition of tumour growth and higher overall survival in cancer patients [96], while other studies indicate that IDO is often related to cancer progression, formation of metastatic niches, downregulation of T cell activity, higher frequencies of liver and pulmonary metastasis, and reduced TIL and NK cells in human cancers [92]. This evidence concerns the gene IDO1 and cancer.