A review. Recombinant proteins are often expressed with an addnl. polypeptide the role of which is to aid purifn., detection, or functional studies. Recently, the role of these tags has been extended to mediate the labeling of the protein of interest with chem. diverse compds. These approaches are of importance for protein science as they allow proteins to be equipped with properties that cannot be genetically encoded and also enable the use of a single fusion protein for several different applications. Several new approaches have been developed for the covalent labeling of fusion proteins both in living cells and in vitro. [on SciFinder (R)]