The oxidation of Cys leading to the formation of the disulfide bond is a well known post-translational modification in biochemistry. Such bridges play a pivotal role in protein folding, regulation and stability. However, recently a new covalent cross-link other than the disulfide bond was reported. The newly characterized cross-link is established between Cys and Lys residues which are shown to be linked through an O atom. The bond was coined as the NOS bond and it was later on shown to take place in a wide variety of proteins, with wide ranging implications for regulation and catalytic mechanisms. It is remarkable that this bond is formed upon oxidizing conditions and that the reduced Cys and Lys residues can be fully recovered under the action of reducing agents.