IDO1 and gastric cancer: We previously reported that the expression of immune molecules such as programmed death 1 (PD-1), programmed death-ligand 1 (PD-L1), and indoleamine-pyrrole 2,3-dioxygenase (IDO) is associated with poor prognosis and immunotolerance through induction of the activation of regulatory T cell (Treg cell) in gastric cancer and CRC [9–11].