Reactive oxygen species (ROS) react with proteins, resulting in protein modification, such as protein sulfenylation through the reversible oxidation of cysteine residues. Redox-sensitive cysteine residues in proteins can function as sensors of ROS and serve as molecular switches, activating or deactivating proteins, following a change in oxidative state. However, the accumulation of proteins with irreversible cysteine oxidation is a hallmark of stress-induced cellular damage associated with diseases like cancer. Cayman’s Sulfenylated Protein Cell-Based Detection Kit employs the cell-permeable and chemoselective DAz-2 probe to detect sulfenic acid-modified proteins in living cells. It can be used to monitor intracellular sulfenylated protein levels in living cells to discriminate between normal and pathological conditions or to help identify new pathways regulated by sulfenic acid formation.