Two Weeks Later, Cracks in a Carefully Crafted Policy

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President Bush's decision to fund work on some 60-odd existing colonies of human stem cells, and only those colonies, hit a serious snag last week with the revelation that virtually all stem cells are cultivated using embryonic mouse tissue. The mouse cells provide the human ones with nutrients and growth factors crucial to their survival and proliferation. The problem: under FDA rules, mouse-fed stem cells given to treat human patients would be considered a "xenotransplant," or tissue from another species. Although hundreds of patients have received liver and fetal cells from pigs without any sign of foreign infection, the agency could halt a stem-cell procedure if it felt the human patient was at risk of getting an animal virus. The news sparked renewed calls for the President to loosen his policy and allow further harvesting from embryos, this time without using animal tissue in the Petri dish. Senator John Kerry warns that if federal dollars aren't made available for new cell lines, Congress may yet wrest the policy from the White House. Just when you thought the fight was over.

--Reported by Frederic Golden/New York