A signal peptide (15–30 amino acids at the N-terminus) contains a hydrophobic core that is recognised by the signal recognition particle (SRP) or the SecYEG translocon. It directs the protein to the ER membrane (eukaryotes) or inner membrane (bacteria) for co-translational or post-translational translocation into the secretory pathway. Signal peptidase cleaves the signal peptide during translocation, releasing the mature protein into the periplasm or extracellular medium.