The endoplasmic reticulum (ER) is involved in various cellular functions. The rough endoplasmic reticulum (RER) is characterized by the presence of ribosomes on its surface and is primarily responsible for the synthesis and modification of proteins destined for secretion, insertion into membranes, or delivery to other organelles. The smooth endoplasmic reticulum (SER) lacks ribosomes and is involved in lipid synthesis, steroid hormone synthesis, detoxification, and calcium storage. Protein synthesis is mainly carried out by ribosomes, either free in the cytoplasm or attached to the RER. Therefore, synthesis of protein is not a primary function of the smooth endoplasmic reticulum.