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The Rab family of proteins is one of the Ras family of monomeric G proteins. There are approximately 70 types of known Rabs in humans. Rab GTPases are involved in the regulation of vesicular traffic in eukaryotic cells, among which vesicle formation, vesicle movement along actin and tubulin network and membrane fusion. Rab proteins are bound at the surface of the membrane by a lipid group covalently linked to an amino acid. As Rabs are GTPases, they have two conformations. The inactive form is bound to GDP and the active one is linked to GTP. Rab escort proteins only bound Rab-GDP and Rab effectors only bound Rab-GTP. Rabs work thanks to Rab effectors.

Rab27A and Rab27B are isoforms (72% of identity) and use several specific effector proteins, among which the Exophilin3/Melanophilin/Slac2-a, to regulate the exocytosis of secretory granule cells. Two groups of Rab27 effectors can be made, depending on their interactions specificities. The first one is composed of Melanophilin, Exophilin4, Exophilin5 and Exophilin6, which are specific effectors for Rab27. The other group consists of Rabphilin-3a, Granuphilin-a and JFC1, which are also effectors for Rab3 and Rab8 for instance. Rab27A/B proteins are not only involved in the transport of lysosome-related organelles regulation and may regulate more types of granule exocytosis mechanisms, like endocrine.