PDCD1 and neoplasm: In our current study, CD39 but not PD-1 expression was observed in FoxP3−Helios+ and FoxP3+Helios+ Treg subsets in normal tissue, whereas within TT, co-expression of PD-1 and CD39 was observed in all the three Treg subsets, indicating that CD39 expression alone is not sufficient for Treg-mediated immune suppression, but its co-expression with PD-1 is required for these Treg subsets for tumor-specific immune suppression.