Stem cell researchers maintain that whatever the potential of adult stem cells, embryonic research could help them achieve it. Last December, for instance, Chien's group reported finding markers for heart muscle stem cells by tracing their development from embryonic tissue.

Similarly, the three research teams that last week reported turning mouse skin cells into embryolike cells say they will have to study embryonic cells to learn how to reprogram human cells in the same way and to understand their potential.

Update: On June 13, Nature published a corrected version of the figure. The journal said it checked the data with independent experts, "who concur that the data in Figure 1 are flawed, but agree that [the retraction] does not negate the central findings of the paper."

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