ER signal sequence

An ER signal sequence is polypeptide chain targeting information recognized by machinery delivering a protein to the endoplasmic reticulum. Many soluble cargoes have a removable N-terminus signal; membrane insertion can use internal signals. An ER signal sequence alone does not specify the final destination.^[ref]

For an original counting example, stipulate an unbranched precursor polypeptide chain of $180$ amino acid residues whose first $20$ amino acid residues form a cleaved ER signal sequence. With no further processing, the remaining chain has $180-20=160$ amino acid residues and $159$ peptide bonds. The initial chain had $179$ peptide bonds; the detached signal has $19$, so $159+19=178$ remain after one bond is cut. The check counts adjacent positions: an unbranched chain of $n$ amino acid residues has $n-1$ peptide bonds. These stipulated lengths are not a natural sequence, a measured size or evidence that a particular sequence functions as an ER signal sequence.