In biochemistry, S-nitrosylation is the covalent attachment of a nitric oxide group (?NO) to a cysteine thiol within a protein to form an S-nitrosothiol (SNO). S-Nitrosylation has diverse regulatory roles in bacteria, yeast and plants and in all mammalian cells. It thus operates as a fundamental mechanism for cellular signaling across phylogeny and accounts for the large part of NO bioactivity.

Reference
Wiki: S-Nitrosylation