The Ras-related superfamily of guanine nucleotide binding proteins includes the R-Ras, Rap, Ral/Rec and Rho/Rab subfamilies all of which are thought to play an important role in either endocytosis or in biosynthetic protein transport. The process of transporting newly synthesized proteins from the endoplasmic reticulum (ER) to various stacks of the Golgi complex and to secretory vesicles involves the movement of carrier vesicles and requires Rab protein function. Rab proteins are also an integral part of endocytic pathways. Rab 40A, Rab 40B and Rab 40C each contain one SOCS box domain and are members of the Rab subfamily of GTPases. Localized to the cytoplasmic side of the cell membrane, Rab 40A, Rab 40B and Rab 40C are thought to function as substrate-recognition components of a SCF-like ECS (Elongin-Cullin-SOCS-box protein) E3 ubiquitin ligase complex that functions to mediate the degradation of target proteins.