IDO1 and neoplasm: Clinical success using immune checkpoint inhibitors has led to the identification of additional checkpoints that mediate tumor immunosuppression, such as the lymphocyte-activation gene 3 (LAG3), T cell immunoglobulin and mucin-domain 3 containing-3 (TIM3), Siglec-15, indoleamine 2, 3-dioxygenase 1 (IDO1), and glucocorticoid-induced tumor necrosis factor receptor (GITR).