ENPP1, also known as PC-1 or Ly-41, is a 115 kD type II transmembrane enzyme that is located on the cell's surface as a homodimer. ENPP1 hydrolyses ATP and other nucleoside 5' triphosphates to produce pyrophosphate which is an inhibitor of calcification and bone formation. It is involved in nucleotide recycling, cell adhesion, and regulates the signaling of insulin and purinergic receptors. ENPP1 is expressed by plasma cells, subsets of T and B1 cells, chondrocytes, capillary endothelium of the brain, renal distal tubules, and hepatocytes.