Nuclear intermolecular duplexes appear to be a general feature of nucleated cells. Most of these duplexes are formed between large RNA as well as between large and small RNA molecules. A significant portion of the large molecules belong to a special class of RNA that is restricted to the nucleus and, therefore, not designated for export. These molecules are assembled with proteins and form a structure of a higher order. The possibility that these molecules and a set of small nuclear (...) RNAs are components of a complex machine, “the transportosome”, which functions in nucleocytoplasmic traffic, is discussed. (shrink)