IDO1 and neoplasm: In this regard, inhibitors of indoleamine 2, 3‐dioxygenase (IDO) are under clinical investigation, particularly in combination with PD‐1/PD‐L1 inhibitors because of the observed correlation between the expression of PD‐1 and IDO immune checkpoints in patient‐derived tumor tissues and the possible role of IDO in inducing resistance to inhibitors of the PD‐1/PD‐L1 pathway.8, 9