Cyclic AMP (cAMP) or cyclic adenosine monophosphate acts as a secondary messenger and is used in cell signalling. The concentration of cAMP in cytosol can be increased via an extracellular signal. It can be deactivated by phosphodiesterase (PDE) into AMP. In most animal cells, cAMP can activate protein kinase A (PKA) by binding to it's regulatory subunits which activates and release its active catalytic subunits. These active catalytic subunits can then go on to phosphorylate other proteins, creating a signalling cascade. In unstimulated cells, levels of cAMP are kept low by phosphodiesterases in order to keep the bound inactive. ATP is converted to cAMP by adenylyl cyclase, cAMP is then used in a range of signalling pathways.