The green revolution Green fluorescent protein allows gene expression and protein localization to be observed in living cells.

From Leuwenhoek's light microscope to the development of the electron microscope, immunofluorescence and video microscopy, advances in cell biology have often come about through the development of new technologies that allow us to see better what is happening inside cells. I will give two examples. First, gene expression has progressed from something that could only be assayed for a population of cells in a test tube, as studied by Jacob and Monod, to something that can be assayed at the single cell level through the use of in situ hybridization and immunofluorescence techniques. This has allowed the study of the developmental biology of multicellular organisms at a new level of detail. Second, the ability to observe the behavior of molecules within living cells by video microscopy has changed the way that we think of cells. For example, the cytoskeleton was until recently thought to be a static structure, a 'skeleton' seen in fixed samples. Video microscopy has demonstrated that the cytoskeleton is actually a highly dynamic structure, changing during the cell cycle, cellular morphogenesis and development, and in response to environmental cues. In part, this revolution in understanding has come about through the ability to isolate the major structural proteins of the cytoskeleton, to covalently label these proteins with fluorescent molecules, and to introduce the labeled proteins into cells. This has allowed the direct observation of cytoskeletal dynamics by fluorescence microscopy. Although this technique could, in theory, be applied to any protein of interest, it has several limitations. It requires that the protein be purified in quantity, that the protein maintain its function after covalent modification with the fluorescent molecule, and that the