While CD39 expression is often associated with Tregs, their coexpression of CD73 is controversial.18 Gourdin et al have shown that in human breast cancer, Tregs are CD39+ and CD73−, and instead found CD73 expression in a subset of effector CD4+ T cells.19 Similarly, we confirm that FOXP3+ are CD39+ (online supplemental figure S4A)10 13 and we see a significant enrichment of CD39+Foxp3+ cells in the tumor tissue (online supplemental figure S4B), yet see little CD73 expression among CD4+ T cells in the tumor (figure 1C). The gene discussed is ENTPD1; the disease is breast cancer.