Endoplasmic reticulum -what is the function

The Endoplasmic Reticulum is a type of organelle in Eukaryotic cells that forms an interconnected network of flattened, membrane-enclosed sacs or tube-like structures known as cisternae. There are two types, rough and smooth, the outer face of the rough Endoplasmic Reticulum  studded with Ribosomes that are the sites of protein synthesis. The main function of them is protein transport., which are transported throughout the cell, and are marked with an address tag called signal sequence.