Example of an SDS non-reducing acrylamide gel of an LF_N cysteine mutant (I77C) reacted with MTS reagents, followed by incubation with maleimide 5,000 PEG. Lane 1, I77C; lane 2, I77C plus maleimide PEG; lane 3, I77C reacted with MTS-ES, followed by maleimide PEG; lane 4, I77C reacted with MTS-ET, followed by maleimide PEG; lane 5, I77C reacted with MTS-ACE, followed by maleimide PEG. Note the gel shift in lane 2 caused by the reaction of the maleimide PEG with the cysteine on I77C; the virtual absence of any such shift in lanes 3–5 indicates that almost all the I77C had reacted with the MTS reagent.