Reprogrammed: Scientists have genetically modified skin cells so that they behave like embryonic stem cells, which can develop into virtually every tissue type in the body. Credit: Junying Yu/University of Wisconsin-Madison

On the second floor of a building in one of South San Francisco's numerous business parks, a new biotech company has set up shop. The walls sport a fresh coat of white paint, and the bench tops are shiny and bare. The tile floors are still glossy, and an expensive new cell-sorting machine sits, untouched, on the loading dock downstairs.

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