HAVCR2 and glioblastoma: Other possible targets for checkpoint inhibitor therapy in glioblastoma include Indoleamine-pyrrole 2,3-dioxygenase (IDO), T-cell immunoglobulin mucin-3 (TIM-3), Killer-cell immunoglobulin-like receptors (KIRs), and 4-1BB (for activation rather than blockade), all of which have been investigated to varying degrees [216,218,219,220,221,222].