Rab proteins are low-molecular-weight GTP-binding proteins that form the largest branch of the Ras superfamily of GTPases. Located on the cytoplasmic face of organelles and vesicles, rab proteins are involved in intracellular membrane fusion reactions. Three membrane proteins, synaptosomal associated protein of 25 kDa (SNAP-25), synaptobrevin, and syntaxin, form the core of a ubiquitous membrane fusion machine that interacts with the soluble proteins N-ethylmaleimide-sensitive factor (NSF) and a-SNAP. Rab proteins, in coordination with the core fusion machinery and Munc-18, help to mediate vesicle docking and fusion. There are over 40 Rab proteins in mammals. Rab11a and Rab11b are known markers for protein trafficking, sorting, and recycling in the endosomal pathway. The Rab11 proteins are enriched in recycling endosomes and the trans-Golgi network, where they regulate membrane recycling back to the plasma membrane. Rab11a is ubiquitously expressed, while Rab11b is expressed mainly in the heart and brain.