Figure 1. Intracellular trafficking through the endo/lysosomal system. The internalized cargo is initially contained within endocytotic vesicles which fuse together to become the early endosome. Cargo can be recycled directly back to the plasma membrane from the early endosome through the “short recycling loop” or retained within the maturing endosome. As the early endosome matures, cargo can be trafficked through the trans-golgi network for repackaging or trafficked back to the plasma membrane through the “long recycling loop.” As the early endosome matures to multivesicular bodies (MVB), the retained cargo is internalized into intraluminal vesicle (ILVs) formed within the fluid phase of the MVB and subsequently delivered to the lysosome. A biochemical event that occurs during the maturation of an early endosome to an MVB is the RAB5 to Rab7 switch. Although the precise role that the Rab5 to Rab7 switch is unknown, it serves as a reliable marker for the transition from an early endosome to a late endosome.