Although phosphorylation of activated rhodopsin (MII) reduces transducin activation, complete deactivation occurs only after arrestin (S-antigen or SAG, Yamaki et al. 1988) binds to and sterically caps MII. SAG is capable of binding unphosphorylated MII, but with very low probability. Binding affinity increases greatly upon phosphorylation. Binding of SAG generally occurs after MII has been phosphorylated ~3 times, and binding prevents further phosphorylation. Interestingly, rods express a small amount of a splice variant of SAG, P44, but its function is not yet known. Defects in SAG cause Oguchi type 1 disease (CSNBO1; MIM:258100), a recessive form of congenital stationary night blindness characterized by impaired scotopic vision (Fuchs et al. 1995).