So let's designate this as Er. So what distinguishes the polypeptides destined to remain inside the side of plasma, the cell, compared to those that are ultimately secreted or end up being inside the plasma membrane of that cell? So, basically, it's the sequence of amino acids known as the signal sequence. So a polypeptide that is destined for secretion or to be inside the cell membrane contains a special sequence of amino acids beginning at the beginning of that growing polypeptide chain, at the end terminus. And as soon as the ribosome synthesizes the signal sequence, a group of molecules known as the signal recognition particles, or SRP, can recognize that sequence. So as soon as the signal sequence is synthesized by the free ribosomes, the signal recognition particles, the complex of proteins and RNA molecules, binds to the signal sequence and moves the ribosome to the er membrane.