In the first experiments, you treat the denatured proteins with radiolabeled NEM, then break any disulfide bonds with a reducing agent dithiothreitol (DTT) and react a second time with unlabeled NEM.

In the second experiment you do the reverse: you first treat the denatured proteins with unlabeled NEM, then break disulfide bonds with DTT and treat with radiolabeled NEM. The proteins are separated according to size by electrophoresis on a polyacrylamide gel