Phosphodiesterases (PDEs, also designated cyclic nucleotide phosphodiesterases) are important for the downregulation of the intracellular evel of the second messenger cyclic adenosine monophosphate (cAMP), as they are responsible for hydrolyzing cAMP to 5'AMP. PDE6G, also designated phosphodiesterase 6G cGMP-specific rod g, is an oligomer composed of two catalytic chains (a and b), an inhibitory chain (g) and an d chain. PDEG6G functions in the processes of transmission and amplification of the visual signal. A mutation in the rod PDE-g gene desensitizes and delays murine rod photoreceptors. PDE6H, also designated phosphodiesterase 6H cGMP-specific cone g, is a tetramer composed of two catalytic chains (a and b) and two inhibitory chains (g). It functions similarly to PDE6H in vision processes. Defects of the PDE6H gene cause retinal cone dystrophy 3 (rcd3), also designated cone dystrophy with night blindness and supernormal rod responses.