One major antimicrobial effector mechanism is the tryptophan-degrading enzyme indoleamine 2,3-dioxygenase (IDO), which can be induced in many human cell types, including myeloid cells, fibroblasts, epithelial cells and tumour cells upon stimulation with the pro-inflammatory cytokine interferon-γ (IFN-γ) by JAK-STAT signalling. The gene discussed is IDO1; the disease is neoplasm.