Nucleotides,the structural subunits of the nucleic acids,are also important extracellular signaling molecules. P2 receptors are a family of cell surface receptors that mediate a wide variety of physiologic effects in response to extracellular nucleotides. These receptors fall into two classes: P2X receptors,which are ligand-gated ion channels that mediate calcium and potassium fluxes in response to ATP,and P2Y receptors,which are G protein-coupled receptors (GPCRs). P2RX4 (P2X purinoceptor 4) is a receptor for ATP acting as a ligand gated ion channel.