Cyclic nucleotides can be found in many different types of eukaryotic cells, including photo-receptor rods and cones, smooth muscle cells and liver cells. Cellular concentrations of cyclic nucleotides can be very low, in the 10−7M range, because metabolism and function are often localized in particular parts of the cell.[1] A highly conserved cyclic nucleotide-binding domain (CNB) is present in all proteins that bind cNMPs, regardless of their biological function.