• The rough endoplasmic reticulum (RER) is the primary site for the synthesis of proteins that are destined for secretion, insertion into membranes, or delivery to other organelles like lysosomes.
• The RER's surface is studded with ribosomes, which are the cellular machinery responsible for translating messenger RNA (mRNA) into protein chains.
• As ribosomes synthesize these proteins, they are threaded into the lumen (internal space) of the RER.