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It didn't take long after Dolly the sheep was cloned in 1996 for maverick scientists to start talking about cloning humans.

One, Severino Antinori, attracted global condemnation when he announced plans to use cloning to help infertile couples. Critics of the idea argued that vital questions had to be answered first: about the health of the progeny, their identity and, for those so inclined, the spiritual dimensions.

Those clones never arrived: the technology was not up to it. But now comes news that biologists have turned mature animal cells into something resembling "totipotent" stem cells – and are trying to do the same with human cells (see "Stem cell power unleashed after 30 minute dip in acid").

Totipotent cells are the most versatile of all stem cells; a single one can develop into an embryo with a placenta, and hence give rise to a fully formed animal – in other words, a clone. The researchers do not use the word "totipotent" and have no stated intention of cloning a human. But the research seems to have reopened the...