IDO1 and neoplasm: In extra-hepatic bile-duct carcinoma, elevated expression of indoleamine 2,3-dioxygenase 1 (IDO1) is significantly correlated with reduced infiltration of CD8+ tumour-infiltrating lymphocytes and poorer clinical outcomes, supporting the concept of a kynurenine–aryl-hydrocarbon-receptor (AhR) driven immunotolerant programme.