= Signal peptidase
{wiki}
A <signal peptidase> removes a cleavable <signal peptide>; a suitable cleavage site and access are required.<Molecular Biology of the Cell, 4th edition by Bruce Alberts and others/The Endoplasmic Reticulum>[Molecular Biology of the Cell, 4th edition by Bruce Alberts and others; “The ER Signal Sequence Is Removed from Most Soluble Proteins After Translocation”: adjacent cleavage-site requirement]{ref}
For the stipulated $180$-<amino acid residue> precursor under <ER signal sequence>, removal of the first $20$ <amino acid residues> leaves $160$. Now stipulate that positions $101$–$120$ of that precursor are instead retained as a <stop-transfer sequence>. Their positions in the mature chain become $81$–$100$, because the numbering origin changes when the first $20$ positions are removed. The mature chain still has $160$ <amino acid residues>, but its membrane-spanning segment is retained. A second hypothetical chain with an uncleaved internal <signal anchor> loses no positions at all. These are conditional bookkeeping examples, not predictions from invented biological sequences.
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