Compartmentalised cAMP SignallingCharacterisation of the cyclic AMP signalling pathway in glycogenolysis as the first "second messenger" system opened the door for study of "cellular signalling" as a Depending on receptor type, cAMP produced at the cell membrane can reach far into the cell or stay localised to its site of production. The distance cAMP travels and its ability to form three-dimensional gradients, triggering activation of cAMP effectors, are determined by the phosphodiesterase (PDE) landscape [8]. Often, PDEs are expressed at low levels yet it is their localisation to demarcated positions within cells that underpins both their effectiveness and function. Indeed, reporter technology devised to visualise cAMP gradient formation following specific receptor activation [9] has confirmed that PDE positioning is crucial to the formation of multiple, simultaneous and spatially distinct cAMP gradients that drive defined physiological responses. PDE familiesPDEs are a vast super-family of distinct, highly regulated enzymes which can be classified based on primary structure into class I, II and III -the largest, class I,