So let's begin by briefly discussing cysteine proteases. So in cysteine proteases, we also have amino acids in the active side that work together to basically catalyze the cleavage of peptide bonds. But unlike encryption, proteases insisteine proteases, it's the cysteine residue found on the active side that acts as that nucleophilic agent that will cleave that peptide bond. 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.