A classical secretory protein vary from a cytosolic protein by having a sequence about 13-35 amino acids long at its N-terminal end called as a signal peptide or signal sequence. The signal peptides of various secretory proteins vary in amino acid sequence but there are some regular features, for instance the center of the sequence commonly having of 10-15 hydrophobic amino acids. An signal hypothesis was proposed from early work in this area and predicted in which the signal peptide directs the secretory protein to the RER membrane and so goals the protein to cross into the RER lumen and be exported. The signal hypothesis has been explained to apply to protein secretion in plant, animal and bacterial cells.