1. A sequence of amino acids at the N-terminus of a protein that indicates that further synthesis should occur on membrane-bound ribosomes (rough endoplasmic reticulum) and that the proteins are to be integral membrane proteins or are to be processed through the Golgi and secreted. The sequence (usually hydrophobic but with some positively charged residues and ~20 aa long) is synthesized in the cytoplasm and is recognized by the signal recognition particle which blocks further translation until the ribosome has bound to the signal recognition particle receptor. The signal sequence is later removed by the signal peptidase complex in the lumen of the ER.
2. Any sequence that determines post-translational uptake by organelles (e.g. KDEL motif).