IDO1 and neoplasm: Since VNP20009 preferentially accumulates in tumor at a ratio greater than 1000/1 compared with normal organs, it is widely used as a delivery vehicle expressing exogenous proteins such as TNF-related apoptosis-inducing ligand (TRAIL), Histidine-proline-rich glycoprotein (HPRG), or carrying shRNA-expressing plasmid to silence the expression of genes like STAT3 and indoleamine 2,3-dioxygenase (IDO) [40–43].