The binding pose of the ligand is the same as that observed in the doubly occupied enzyme previously solved (adduct root-mean-square deviation of 0.75 and 0.32 Å with 7NF5 chain "A" and 7BE7 chain "A", respectively). The b-factors of chain "B" (not containing the ligand) are larger than those of "A" (Fig. and). A fully consistent picture is obtained by letting the double occupied crystal for two months in their growing solution: co-crystals of Mpro obtained in excess of MG-132 as described in ref. , after two months turned showing a positive Fo-Fc difference map corresponding to the covalently bound ligand only in one chain while the other resulted empty (Fig. ). The single occupied site crystals diffracts to resolutions similar to those of fresh crystals, around 1.85 Å. This shows that only one binding site remains occupied if the enzyme is allowed to have enough time to let one ligand break its covalent bond and diffuse. The results strikingly differ from freshly obtained crystals prepared with the same protocol, that clearly showed to have both sites occupied 36 .