And this is found inside the active side. And so that implies that it's this Serene molecule. And the serine molecule has a number of 195. So if we count all the amino acids, beginning with the first one along the entire polypeptide chain, this Serene will be labeled as 195. So among the 28 serum residues found on China Trypsin, only Serene 195 actually reacted with this irreversible inhibitor di isopropyl phosphoridate. And so this implied that serine 195 must be the one that plays a major catalytic role in the active side of that enzyme.