Please note that the content of this book primarily consists of articles available from Wikipedia or other free sources online. A signal peptide is a short (3-60 amino acids long) peptide chain that directs the transport of a protein. Signal peptides may also be called targeting signals, signal sequences, transit peptides, or localization signals. There is some confusion relating to the precise meaning of the term ''signal peptide''. Some sources refer to signal peptides as only the pre-sequence that targets the propeptide to the endoplasmic reticulum and through the secretory pathway. Used in this system it does not refer to ''transit peptides'' . A ''transit peptide'' used in this system refers to the part of the pre-sequence that targets the protein to other organelles, such as mitochondria, chloroplasts and apicoplasts.