IDO1 and neoplasm: Unfortunately, hypoxia and soluble factors produced by tumor/tumor-associated cells (such as IDO-derived L-kynurenine, TGF-β, and PGE2) [38,39,40,41] and soluble forms of NCR-ligands shed from tumor cells surface (such as the soluble form of either BAT3 or B7-H6) [30,42,43] can impair NCR expression and function.