PDCD1 and neoplasm: In patients with breast or colon cancer, the expression of the immune checkpoint component PD-1 on CD8+ T cells showed a strong positive correlation with the concentrations of Kyn in blood and tumour tissue.55 Collectively, these studies provide evidence of the role of Trp catabolism in promoting an immunosuppressive tumour microenvironment, thus confirming the importance of IDO1 as an immunotherapeutic target.