Two Weeks Later, Cracks in a Carefully Crafted Policy

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...

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