IDO1 and cancer: Apart from expression by immune cells, the majority of cancers also express IDO1 and this fact is exploited during the process of tumour immunoediting, during which enzyme expression is increased resulting in a rapid acceleration of immunosuppression and tumour growth Human metabolomics studies are concordant with these pre‐clinical findings and an elevated Kyn/Trp ratio or increased IDO activity have been associated with primary CIT resistance, poor prognosis and early progression of cancer.20, 21, 52