A review of recent developments in the labeling of fusion proteins with synthetic mols. in the living cell or in complex mixts. The most commonly used tags are presented and then focuses on a new class of tags that are not limited to their genetically encoded function but rather serves as general acceptors for synthetic mols. The tetracysteine tag, which reversibly binds to biarsenical compds., and the human O6-alkylguanine-DNA alkyltransferase tag, which is irreversibly alkylated by benzylguanine derivs., are prototypes of this approach. Both provide the resp. fusion protein with functionalities that can not be genetically encoded and thereby complement the properties of the traditional fusion proteins. [on SciFinder (R)]