The small GTPase Rab5 regulates membrane traffic into and between early endosomes by specifically recruiting cytosolic effector proteins to their site of action on early endosomal membranes. Rab5 occupies a restricted membrane subdomain on endosomes that has distinct biochemical features when compared with neighboring subcompartments (Sonnichsen et al. 2000). Rab5 is believed to form this subdomain by recruiting the specific PI3 kinase VPS-45, causing localized production of PI-3-phosphate (PI-3P) (Christoforidis et al. 1999). Rabenosyn-5 is a Rab5 effector, recruited in a phosphatidylinositol-3-kinase dependent fashion to early endosomes where it serves as a molecular link between VPS-45 and Rab5.