Besides, TAMs inhibit the cytotoxic functions of T cells, natural killer T cells, and NK cells through the expression of the ligands for the immune checkpoint receptors programmed cell death-1 (PD-1) and cytotoxic T lymphocyte-associated protein 4 (CTLA-4) [121], which are highly expressed on the surface of CSCs in various cancer types, allowing CSCs to escape and avoid elimination by the immune cells [148]. The gene discussed is PDCD1; the disease is cancer.