Endosomes are formed by the invagination of the plasma membrane to form vesicles in an effort to recycle components of the cell. Endosomes can be coated in clathrin when vesicles form at clathrin-coated pits. Caveolins are 21-24kD integral proteins that interact with cholesterol and are the main structural components of the cholesterol/sphingolipid- enriched plasma membrane caveolae. Each stage of endosome maturation is marked by a unique set of proteins. EEA1 is an early endosome marker that is essential for membrane fusion and trafficking. Members of the ras superfamily of small Rab GTPases, specifically Rab5, Rab7, and Rab11 are markers of the early, late and recycling endosomes.