Modification of proteins by ROS. ROS can oxidize specific reactive cysteine (Cys) residues within target proteins generating sulfenic form (RSOH) of proteins. RSOH is unstable and can be further oxidized to sulfinic form (RSO2H). Under greater oxidative stress, sulfonic form (RSO3H) can be generated. Although the formation of RSOH and RSO2H is reversible, generation of RSO3H is irreversible.