Conversely, several factors contribute to NK cell dysfunction, such as severe hypoxia, the accumulation of tumor cell metabolites, alterations in inhibitory receptors or NK inhibitors, and the presence of molecules like TGFβ, prostaglandin E2 (PGE2), or indoleamine 2,3-dioxygenase 1 (IDO1). This evidence concerns the gene IDO1 and neoplasm.