In biochemistry, tyrosine sulfation is a posttranslational modification where a sulfate group (−SO3) is added to a tyrosine residue of a protein molecule. Secreted proteins and extracellular parts of membrane proteins that pass through the Golgi apparatus may be sulfated. Sulfation was first discovered by Bettelheim in bovine fibrinopeptide B in 1954[1] and later found to be present in animals and plants but not in prokaryotes or in yeast.