Redox-sensitive cysteine residues in proteins may function as sensors of reactive oxygen species (ROS) and also serve as molecular switches, activating or deactivating proteins, following a change in oxidation state. Modification of protein function through the reversible oxidation of cysteine is emerging as a biologically relevant signal transduction mechanism. Sulfenic acid is the initial oxidation product of cysteine by relatively mild oxidizing agents such as hydrogen peroxide. Sulfenic acid can be reduced back to the free thiol or be further oxidized to sulfinic and sulfonic acids. DAz-1 is a cell-permeable chemical probe that reacts specifically with sulfenic acid-modified proteins. The azido group of DAz-1 provides a method for selective conjugation to phosphine- or alkynyl- derivatized reagents, such as biotin or various fluorophores, for subsequent analysis of the labeled proteins. DAz-1 is a less sensitive probe for sulfenic acid detection compared to its analog DAz-2 .