Scientists are responding with a mix of encouragement and criticism to proposed sources of human pluripotent stem cells put forth at a meeting of the President's Council on Bioethics earlier this month that in principle could placate critics of embryonic stem cell research.

"Altered nuclear transfer," proposed by council member William Hurlbut, involves modifying conventional somatic cell nuclear transfer (SCNT) by silencing particular developmental genes in the somatic cell nucleus prior to transfer into the enucleated oocyte. For example, Hurlbut cited Cdx2, mutations in which have been shown in mice to cause death at the blastocyst stage because they fail to form a trophectoderm, which normally gives rise to the placenta. However, these embryos can still give rise to mouse embryonic stem cells.

Silencing of such genes, in principle, would allow cellular development such that embryonic stem cells could be obtained. But because the entity would have no "global,...

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