Distinguishing mechanisms enabling an immunoevasive cancer cell phenotype include a reduced immunogenicity due to loss in expression of tumor-associated antigens (TAAs) or major histocompatibility complex (MHC) class I molecules, acquired DNA copy number alterations and oncogenic signaling, upregulation of cellular immune checkpoints like programmed death ligand 1 (PD-L1), indoleamine 2,3-dioxygenase (IDO), and tryptophan 2,3-dioxygenase (TDO), and altered metabolism resulting in a low pH and secretion of various metabolites4. The gene discussed is TDO2; the disease is cancer.