Signal sequence; the N-terminal portion of a secretory or membrane protein that assists it across the membrane of the rough endoplasmic reticulum, where it is synthesized, but which is cleaved from the protein even before the synthesis of the protein is complete. Mitochondrial and chloroplast proteins that are synthesised on ribosomes also have signal sequences that target them to their organelle; these may be N-terminal or internal, and may or may not be removed by proteolysis.