Retinal rod and cone cGMP phosphodiesterases function as the effector enzymes in the vertebrate visual transduction cascade. This family represents the inhibitory gamma subunit [<cite idref="PUB00008678"/>], which is also expressed outside retinal tissues and has been shown to interact with the G-protein-coupled receptor kinase 2 signalling system to regulate the epidermal growth factor- and thrombin-dependent stimulation of p42/p44 mitogen-activated protein kinase in human embryonic kidney 293 cells [<cite idref="PUB00008679"/>].