Another mechanism of immunosuppression is the expression of immune checkpoint (IC) molecules on GBM-associated TREGs, including programmed cell death protein 1 (PD-1), whose ligand PD-L1 is expressed by GBM cells as well as microglia and TAM, and their interaction promotes TREG activity and consequently the inhibition of the proliferation and function of effector TH cells and CTL [77,78,79]. The gene discussed is PDCD1; the disease is glioblastoma.