Rab27 proteins are members of the Ras superfamily of small Rab GTPases associated with the process of exocytosis, and in particular the exosome secretion pathway. There are two Rab27 isoforms, Rab27A and Rab27B, which are encoded by separate genes and which exhibit distinct subcellular localization and functions. Rab27A colocalizes to CD63+ vesicles, whereas Rab27B is observed in perinuclear vesicles. Targeted knockdown studies suggest that the isoforms control distinct steps of the exosome secretion pathway. Rab27A participates in docking and membrane fusion from multivesicular endosomes (MVEs) to exosomes, whereas Rab27B mediates the transfer of membrane vesicles from the trans-Golgi network to MVEs. It has been suggested that Rab27A and Rab27B may have importance in exosome secretion by cancer cells.