Transducin (Gt) is a heterotrimeric G protein encoded by GNAT genes and is naturally expressed in vertebrate retina rods and cones. There are two types, alpha-1 chain (expressed in rods by GNAT1) (Lerea CL et al, 1989) and alpha-2 chain (expressed in cones by GNAT2) (Morris TA and Fong SL, 1993). Stimulated opsins can act as GEFs for G (t) alpha subunits by replacing GDP with GTP. Consequently, the G (t) alpha subunit is activated and results in the "vertebrate phototransduction cascade" (Chen CK, 2005). Cyclic GMP Phosphodiesterase is activated which lowers cGMP levels (an intracellular second messenger molecule). Lower cGMP levels can then lead to the closure of cGMP-regulated Na+ and Ca2+ ion channels and a hyperpolarized membrane potential.