Tumours have been shown to inhibit NK cell function by modulation of expression of activating receptors and cytolytic activity, through release of indolamine 2, 3-dioxygenase (IDO), TGF-β and prostaglandin E2 (PGE2) by malignant cells and Tregs [74, 81, 82]. This evidence concerns the gene IDO1 and neoplasm.