Well, we want to label that initial amino acid. Why do we want to label it? Well, because in the second step, what we're going to do is we're going to basically cleave this peptide bond that is holding this Allen amino acid and this Arginine amino acid. And by cleaving it, we're going to separate it from the rest of the polypeptide. And once we separate it, we want to have a means of separating this amino acid that was cleaved, that was separated and the rest of that polypeptide. And that's why we label it with this phenol ISO thiosyanate molecule.