Prior to mutagenesis of the OPH residues, we wanted to predict the correct binding mode of paraoxon in the OPH active site. As described in the Methods section, we performed docking using three pieces of docking software and selected several representative docking poses. Then, we ran short MD simulations on the selected poses to assess their stability in the OPH active site. For this purpose, we calculated the RMSD of the poses at the end of the 15 ns long MD trajectories with respect to the starting poses before MD, and found that the pose with the lowest RMSD value (1.77 Å) was generated by HYBRID (Figure ). Interestingly, we found that the docking pose generated using POSIT, which best overlays the structure of diethyl 4-methylbenzylphosphonate, is not stable and scores an RMSD of 6.6 Å (Figure ). Essentially, while the ligand can dock in an overlapping pose, our simulations indicate it is not stable in that binding mode and that other binding modes are more favorable/stable. In contrast to previous assumptions, this finding suggests that the binding mode of paraoxon is different than that of 4-methylbenzylphosphonate.