Proteins that regulate mRNA metabolism are often bipartite: an RNA binding activity confers substrate specificity, and a "functional" domain elicits the biological outcome. In some cases, these two activities reside on different polypeptides that form a complex on the mRNA. Regardless, experimental separation of RNA binding from function facilitates analysis of both properties, liberating each from the constraints of the other. "Tethered function" assays bring a protein to a reporter RNA through a designed RNA-protein interaction. The function of the tethered protein-whether that be stability, translation, localization, or transport, or otherwise-is then assessed. We refer to this approach as a "tethered function" assay, since it can be examined. The approach does not require knowledge of the natural RNA binding sites, or of the composition of the naturally occurring protein complexes. It can be useful in dissecting how proteins that act on RNAs work, and in identifying active components of multiprotein complexes. RNA-binding proteins previously have been linked to foreign RNA-binding specificities, for a wide variety of purposes. We emphasize here the particular value of tethering to the 3' untranslated region of eukaryotic mRNAs, and the opportunities it presents for the analysis of how those mRNAs are regulated. We discuss experimental design, as well as published and potential applications. (c) 2002 Elsevier Science (USA).