Now let's move on to the second type of category known as substrate analogs or more or more commonly affinity labels. So the thing about these affinity labels is the structure of that particular irreversible inhibitor actually resembles the structure of the natural substrate that binds into the active side of that enzyme. So these irreversible inhibitors are molecules that resemble substrates and this allows them to actually fit into the active side of that enzyme. And once inside, they essentially react in a covalent manner. They modify the residues found inside the active side in a covalent manner and that inhibits that enzyme's activity. Now, what's one example of an affinity label?