ENPP-4 (Ectonucleotide pyrophosphatase/phosphodiesterase 4; NPP4) belongs to a group of ecto-enzymes which regulate the availability of extracellular nucleotides. ENPP-4 hydrolyzes phosphodiester bonds in nucleotides with a preference for adenine nucleotides. It cleaves the diadenosine compounds Ap3A and Ap4a which are released from the dense granules of thrombin-activated platelets. These reactions generate AMP and ADP from Ap3A cleavage, and AMP and ATP from Ap4A cleavage. ENPP-4 is expressed on the surface of vascular endothelial cells where its activity prolongs platelet aggretation and contributes to thrombus formation.