So, we have the mRNA molecule that is being read by the ribosome as shown. And as it's being read, our ribosome is producing our polypeptide chain as shown. So the signal sequence is a special sequence of amino acids that are found at the beginning, at the end terminus of the polypeptide chain. And if the polypeptide chain actually contains the signal sequence, what happens is a special complex of molecules known as the signal recognition particles or SRP, which are basically a complex of RNA and protein molecules, will bind onto the signal sequence. And once they bind onto the signal sequence, they will basically transport this entire complex, known as the SRP ribosome complex, onto the membrane of the endoplasmic reticulum. So once again, polypeptides variedestine for secretion or to be embedded in the cell membrane, begin synthesis in the free ribosome.