Summary cDNA cloning of component A of rat Rab geranylgeranyl transferase confirms identlty of the protein with the human chorolderemla gene product and Its resemblance to Rab3A guanlne nucleotlde dissociation lnhlbltor (GDI), which blnds prenylated Rabs. In blochemical assays we demonstrate that component A binds unprenylated Flab1 A, presents It to the catalytic component B, and remains bound to It after the geranylgeranyl transfer reaction. In the absence of detergents, the reaction terminates when all of component A is occupied with prenylated Rab. Detergents allow multiple rounds of catalysis, apparently by dissoclating the component A-Rab complex and thus allowing recycling of component A. Within the cell, component A may be regenerated by transferring Its prenylated Rab to a protein acceptor, such as Rab3A GDI. In view of its function In escorting Rab proteins durlng and presumably after the prenyl transfer reaction, we propose to rename component A as Rab escort proteln (REP). A genetic defect in REP underlies human chorolderemia, a disease of retinal degeneration.