Returning to the evaluation of the norbornene probes, proteomics analysis was used to confirm the site of labeling for representative hits. Accordingly, after labeling cell lysates with probe 2, the 45 kDa band was excised, digested with chymotrypsin, and analyzed by LC-MS/MS (S25). This band was selected because it was a prominent signal for both probes 1 and 2 in the western blot. The expected product formed from the reaction of 2 with cysteine sulfenic acid was indeed detected on two separate actin proteins (S25), providing additional confirmation of the site-and chemoselectivity of the norbornene probe.