Exosomes exhibit certain advantages over conventional delivery systems. They are natural transporters and hence less likely to be toxic or to cause immune responses. Moreover, exosomes can cross biological barriers such as the blood-brain barrier. The unique membrane composition of exosomes is key to their ability to enter target cells. The membranes of exosomes are enriched in cholesterol and phosphatidylserine. 296 However, vesicles comprising only the lipids from the exosomal membrane are unable to fuse with cells, indicating that exosomal membrane proteins are also important for their activity. A notable advantage of the exosomes as compared to other nanoparticle delivery vehicles is that they do not lead to a harmful accumulation of therapeutic RNAs in the liver.