Common immune checkpoints include PD-1(programmed cell death protein 1), PD-L1(programmed cell death protein 1 ligand), and CTLA-4(cytotoxic T-lymphocyte-associated protein 4), which are often expressed on the surface of T lymphocytes, cancer cells, DCs(dendric cells), Treg(regulatory T lymphocyte), NK cell(Natural Killer cells) and so on (Sangro et al., 2021). Here, CTLA4 is linked to cancer.