And to see how that actually works, let's take a look at the following diagram. So, we have a cysteine residue in the active side of some particular cysteine protease. Now, in the form that we have cysteine here, cysteine is not a strong enough nucleophile. So the sulfur, as shown here, is not a strong enough nuclear file. And it will not be able to attack the carbon of that carbonyl group on that substrate molecule because this simply isn't a good enough nuclear file. So what must happen is another residue must work together with the cystine to basically transform it into a better nuclear file.