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. So how does that actually take place? Well, in a similar way to how it takes place in seren proteases, so we have a nearby residue, for example, a histidine. And on that histidine, we have this nitrogen, which contains a lone pair of electrons.