The work of our laboratory in 1962 led to the evolution of methods for isolation of nucleoli (Busch and Smetana 1970). In the course of these studies it became apparent that the nucleoli were very complex structures that contained a variety of enzymes, nonhistone proteins, the rDNA and precursors of rRNA. It was not known at that time that in addition to many species of macromolecules involved in ribosome synthesis, nucleoli also contained slowly turning over elements.