In vitro co-culture analysis using HCC cell lines suggests that hypoxia increases expression of indoleamine 2, 3-dioxygenase 1 (IDO1) in monocyte-derived macrophages and Tregs leading to reduced CD8+ T cell proliferation and cytotoxic effects and expansion of Tregs (227, 229). The gene discussed is IDO1; the disease is hepatocellular carcinoma.