IDO1 and cancer: Additional checkpoints, including Lymphocyte Activation Gene 3 (LAG-3), T-cell immunoglobulin and Mucin Domain-Containing Protein 3 (TIM-3), T-cell immunoreceptor with Ig and ITIM Domains (TIGIT), Indoleamine 2,3-Dioxygenase 1 (IDO1), and V-Domain Ig Suppressor of T Cell Activation (VISTA), are being explored for their potential in combination therapies, particularly in difficult-to-treat cancers like GBM [35, 36].