In mammalian cells, protein synthesis is compartmentalized; soluble proteins are synthesized on free ribosomes, whereas secretory and membrane proteins are synthesized on endoplasmic reticulum (ER)-bound ribosomes. George Palade and his colleagues in 1960s were the first to demonstrate the role of endoplasmic reticulum in protein secretion. The defined pathway taken by secreted protein is: Rough ER - Golgi - secretory vesicles- cell exterior.