Exosomes, lipid bilayer vesicles secreted from almost all cells, have emerged as a novel signal mediator of cell-to-cell communication. Lipids are essential components of exosomes to outline the boundaries of exosomes and to sequester their components from the external environment. Additionally, recent studies have suggested that exosomal lipids engage in exosome formation, uptake, and signal transduction to receiver cells. Lipids consisting of exosomal membrane are similar to those of cellular

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... membrane, but some lipids seem to be actively enriched in exosomes, indicating a selective lipid loading system during exosome formation. Despite the impor tance and intriguing biological functions, the lipid composition of exosomes is less studied than other components due to technical dif ficulties. Here I summarize current knowledge and limitations of lipid studies of exosomes.