IDO1 and neoplasm: Cancer-associated fibroblasts (CAFs), macrophages and other antigen-presenting cells (APCs), and sometimes tumor cells themselves, upregulate IDO, exacerbating tryptophan catabolism, to create an immunosuppressive tumor microenvironment that prevents T-cell activation by inducing anergy (Box 1) and apoptosis (Moffett and Namboodiri, 2003; Platten et al., 2012).