ENTPD1 and neoplasm: In human and murine liver specimens, CD39 has been observed to be strongly expressed in Kupffer cells and endothelial cells (ECs) of muscularized vessels in the liver.[22] CD39 expression by ECs may directly protect tumor cells from high levels of extracellular ATP, which directly limits tumor cell growth and these antitumor effects could be mitigated by the provision of CD39 or by the intrinsic EC expression of CD39.[23] CD39 expression in Tregs inhibits natural killer (NK) cell activity and is necessary for the growth of metastatic tumors in the liver.[24]