In this context, tumor cells have been shown to express the enzyme indoleamine 2,3-dioxygenase (IDO), which on the one hand depletes tryptophan, a critical amino acid needed for T cell proliferation (4, 5), and on the other hand produces kynurenine, a T cell suppressive metabolic ‘waste’ product (7). The gene discussed is IDO1; the disease is neoplasm.