IDO1 and neoplasm: Recently, researchers identified several other immune checkpoint molecules that are aberrantly expressed in cancers and can hamper antitumor immunity to promote tumor progression via various mechanisms (25–28), such as human endogenous retrovirus-H long terminal repeat-associating protein 2 (HHLA2), B7H3, indoleamine 2,3-dioxygenase (IDO-1) and galectin-9.