PDCD1 and glioblastoma: Other immunotoxins studied in GBM are directed to the surface proteins overexpressed in GBM cells such as EFGRvIII, VEGF, or ephrin receptors [25, 87–89], and their use in combination with other immunotherapeutic tools such as the blockade of checkpoint molecules, PD-1 or CTLA-4, demonstrated tumor clearance in immunocompetent animals [89].