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By Antonio Regalado

A quarter-century ago, researchers isolated powerful stem cells from embryos created through in vitro fertilization. These cells, theoretically able to morph into any tissue in the human body, promised a medical revolution. Think: replacement parts for whatever ails you.

But stem-cell science didn’t go smoothly. Not at first. Even though scientists soon learned to create these make-anything cells without embryos, coaxing them to become truly functional adult tissue proved harder than anyone guessed.

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