In cancer, IDO elevation is often caused as a response to inflammation, specifically by other proinflammatory cytokines, including tumour necrosis factor α (TNFα), interleukin‐1 (IL‐1), IL‐6, interferon‐gamma (IFN‐γ), transforming growth factor beta (TGF‐β), cytotoxic T‐lymphocyte‐associated protein 4 (CTLA‐4) and programmed cell death protein 1 (PD‐1).27, 29. Here, PDCD1 is linked to cancer.