An intricate molecular machine ensures proper sorting and delivery of molecules (protein and lipid) to successive compartments along the endo- and exocytic pathways. The small molecular weight rab GTPases of the ras superfamily constitute key components of this intracellular transport machinery. Rab7 is uniquely poised on late endosomes and our studies on rab7 function provide exciting new information on late endocytic membrane transport. Morphological and biochemical assays demonstrate that rab7 function is required both for the delivery of newly synthesized proteins (from the Golgi) to lysosomes (Press et al., 1998), as well as for the delivery of internalized molecules to compartments containing hydrolytic enzyme activities (Feng et al., 1995; Feng et al., 1999; VSV G Cleavage). Perturbation of rab7 function results in the increased secretion of lysosomal enzymes, including cathepsin D. Increased secretion of such enzymes occurs during normal development and is frequently associated with tumor metastasis, particularly of breast cancers. Mounting an effective immune response critically depends on binding of MHC class II to antigen within endosomes and the subsequent cell surface exposure of MHC class II-antigen complexes. A series of related, collaborative studies reveal a role for rab7-positive endosomes in the formation of MHC class II-antigen complexes (Qiu et al., 1994). Furthermore, transport appears to be modulated through signal transduction cascades (Xu et al., 1996, Figure 7). The combined results indicate that rab7 or associated cofactors may serve as an important cellular switch for modulating the extracellular levels of molecules which can in turn influence cellular interactions and ultimately cell growth properties. More