Wikipedia
Bromobimane
Bromobimane (or monobromobimane) is a heterocyclic compound and bimane dye that is used as a reagent in biochemistry. While bromobimane itself is essentially nonfluorescent, it alkylates thiol groups, displacing the bromine and adding the fluorescent tag (λ = 478 nm) to the thiol. Its alkylating properties are comparable to iodoacetamide.