Inducible T cell costimulatory factor (ICOS, cluster of differentiation, CD278) is an activated costimulatory immune checkpoint expressed on T cells. Its ligand ICOSL was expressed in antigen-presenting cells and somatic cells (including tumor cells in the tumor microenvironment) [PMID: 28713380, PMID: 12800259]. As one of the most common targeted immune checkpoints, ICOS is considered a possible way to develop new immunotherapy [PMID: 29544515]. Some clinical studies shown that blocking ICOS alone can inhibit tumor growth and proliferation, as well as TIGIT, CTLA4, and PD-1 [PMID: 30232519, PMID: 28803728].