IDO1 and neoplasm: In the tumor microenvironment, immunosuppressive myeloid cells upregulate amino acid-metabolizing enzymes, such as Arginase 1 (Arg1) and Indoleamine 2,3-dioxygenase 1 (IDO1), to deplete arginine and tryptophan—amino acids essential for T cell activation and function—thus promoting tumor progression.