P2X receptors are a family of cation-permeable ligand gated ion channels that open in response to the binding of extracellular adenosine triphosphate (ATP). All members of the family are thought to be functionally trimeric. The ionotropic P2X1 receptor has relatively high calcium permeability. It is predominantly expressed in smooth muscle and platelets, but also has a role in synaptic transmission between neurons and from neurons to smooth muscle. Mouse studies suggest that this receptor is essential for normal male reproductive function. ADP has been suggested as a ligand for this receptor but this is no longer widely accepted.