Cyclic nucleotides are produced from the generic reaction NTP → cNMP + PPi,[7] where N represents a nitrogenous base. The reaction is catalyzed by specific nucleotidyl cyclases, such that production of cAMP is catalyzed by adenylyl cyclase and production of cGMP is catalyzed by guanylyl cyclase.[2] Adenylyl cyclase has been found in both a transmembrane and cytosolic form, representing distinct protein classes and different sources of cAMP.[8]