Rab27 is a member of the Ras superfamily of small Rab GTPases implicated in exocytosis . The protein is localized in secretory lysosomes, such as melanosomes in melanocyte or lytic granules in cytotoxic T cells to control exosome secretion pathway. Rab27 has two isoforms, Rab27a and Rab27b. Rab27a colocalizes with part of CD63 staining vesicles, and Rab27b shows perinuclear distribution. Target knock out studies indicate that the isoforms control different steps of the exosome secretion pathway . Rab27a interacts with a wide range of effectors and is involved in multiple steps of exocytosis depending on the effector it associated with and the cell type that is involved . Rab27a has been shown to be an important player in leukocyte function, cancer metastasis and invasion, and insulin secretion.