How Dolly was designed.
Dolly and her fibroblast-derived cousins have changed forever the way we think about animal development. Cells in mature animal tissues are not, as we had thought, irreversibly differentiated. Understanding differentiation is the key to understanding development, and Dolly embodies the extraordinary possibility of manipulating the process--of doing experiments to identify the basic construction rules used in putting animals together.
Andrew Berry is a junior fellow at the Harvard University Society of Fellows. He has a Ph.D. in evolutionary genetics from Princeton.