ENTPD1 and viral infectious disease: In recent years, CD39 has emerged as a marker to identify T cells specific for tumor antigen.3 Unlike PD1, CD39 expression is lacking in bystander CD8+ T cells and highly expressed in CD8+ T cells specific for tumor antigens in colorectal cancer and melanoma.4 5 CD39 expression on CTL also describes a terminally exhausted population in viral infections and tumors.4–8