Abstract: In combination with conventional therapies (e.g., targeted therapy, chemotherapy, and angiogenesis inhibitors etc.), immunotherapies targeting checkpoint molecules have shown promise in the treatment of solid or liquid tumors. However, apoptotic regulatory T cells (Treg) induced by such therapies often become more suppressive in the tumor microenvironment (TME), through increased generation of adenosine tightly controlled by ectonucleotidases, viz. CD39 and CD73. CD39/ENTPD1, a novel checkpoint molecule, is highly expressed and activated on the tumor vasculature and infiltrating immune cells, promoting tumor growth. Deletion or blockade of CD39 enhances anti-tumor activity by augmenting anti-tumor immune responses and inhibiting tumor angiogenesis.