Tregs, myeloid-derived suppressor cells (MDSCs), tumor-associated macrophages (TAMs) and tumor-associated fibroblasts (TAFs) may inhibit NK cell function, either by releasing TGF-β, IL-4 and PGE2, or by contributing to the production of the suppressive factors kynurenine (via IDO) and adenosine (via CD39 and CD73) [191,192]. This evidence concerns the gene IDO1 and neoplasm.