The Rough Endoplasmic Reticulum (RER) is a type of organelle found in eukaryotic cells. It is characterized by its ribosome-studded surface, which gives it a "rough" appearance. The primary function of the RER is to synthesize proteins that are either secreted from the cell, incorporated into the cell's plasma membrane, or sent to an organelle called the lysosome. In addition to protein synthesis, the RER is involved in the initial stages of protein folding and modification. Once proteins are synthesized, they are transported into the lumen of the RER, where they undergo further processing before being sent to their final destinations within the cell or outside of it.