In accord with Gordon Tompkins’ concept, this provides another example of interacting regulatory and catalytic sites, in this case operating across space. These regulatory mechanisms are not mutually exclusive; pyrimidine biosynthesis is controlled by both feedback and repression,15 and cholesterol regulates its availability by mechanisms operating at the levels of gene, enzyme, and intercell transport as discovered by Michael Brown and Joseph Goldstein. Relocation of molecules between subcellular compartments in eukaryoteic cells provides yet another mechanism of regulation. For example, several enzymes involved in DNA synthesis are produced in the cytoplasm at the G1/S-interface and translocate into the nucleus where they form the Replitase multi-protein complex for DNA synthesis mentioned above. 23