Many proteins are modified in the lumen of the endoplasmic reticulum and then transported via vesicles from the rough and smooth endoplasmic reticulum to the Golgi complex, where further modification may take place. In the ER, signal sequences are cleaved away, disulfide bridges are formed, and glycosylation occurs. Sugars, lipids, phosphate or sulfate groups may be added or removed from the proteins passing from one Golgi sac to another.