In tumors, the increase of immunosuppressive cells (Treg cells and tumor-associated macrophages, etc.), the overexpression of immunosuppressive molecules (CTLA-4, etc.), and the decrease of tumor antigens will lead to the inability of CD8T cells to recognize cancer cells, thus promoting the occurrence and development of tumors (Zou and Chen, 2008). Here, CTLA4 is linked to cancer.