And it's the lysine residues on the target protein that are used to actually generate a bond to attach the Ubiquitin to that target protein. So suppose this is our target protein and this here is the side chain group of a lysine residue of a lysine amino acid. So we use this nitrogen on the lysine side chain group to form a bond between this nitrogen and this carbon. And this bond here is known as the isopeptide bond. So the carboxyl terminal group of the ubiquitous is extended and it contains a glycine residue and it's this glycine residue that is covalently attached onto the epsilon amino group of lysine residues found on target proteins. And this bond is known as the isoppeptide bond.