IDO1 and neoplasm: IDO1 and kynurenine corresponded to the strongest GMI in the CAMP, raising two opposing hypotheses on the mechanism underlying the primary source of kynurenine in the tumour microenvironment: either bulk tumour expression of IDO1 is primarily determined by high DC-specific expression or, alternatively, it is driven by a comparatively low expression in far more abundant non-DC-cell populations.