Tumor cells use various tactics to escape NK-cell-induced elimination, directly suppressing NK-activating receptors or releasing immune-suppressive factors such as a transforming growth factor-β (TGF), IL-10, indoleamine 2,3-dioxygenase 1 (IDO1), and soluble NK receptor ligands, including UL16-binding proteins (ULBP) 1–3 and major histocompatibility complex (MHC) class I chain-related proteins (MIC) A and B [11]. This evidence concerns the gene IDO1 and neoplasm.