IDO1 and cancer: PD-L2, Lymphocyte activation gene-3 (LAG3), T cell immunoglobulin and mucin domain (Tim-3), Cluster of differentiation 80 (CD80), Cluster of differentiation 86 (CD86), and Indoleamine 2, 3-dioxygenase 1 (IDO1) are also upregulated, but PD-L1 has received particular interest because the PD-1/ PD-L1 axis is the target of some recent breakthrough cancer therapies[20].