No, Mismatch Endonuclease I is relatively insensitive to salt. However, concentrations of salt > 250 mM can decrease the activity of Mismatch Endonuclease I. It is important to note that Mismatch Endonuclease I is stored in a high salt (500 mM) diluent, so some salt is carried over into the reaction from the storage buffer and should be accounted for in the final salt concentration of any reaction.