Cells export proteins such as insulin, antibodies and digestive enzymes. The recipe leaves the nucleus as mRNA. A ribosome starts reading it in the cytoplasm; the first part of the new chain is a signal sequence that docks the ribosome onto the rough ER, and the chain is threaded inside the ER as it grows. Small vesicles carry the protein from the ER to the Golgi, which modifies and sorts it. A secretory vesicle then takes it to the cell membrane, fuses with it and releases the protein outside (exocytosis). In real experiments, the drug brefeldin A blocks the step from the ER to the Golgi, and exported proteins pile up in the ER.