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President Obama's open-ended order lifting limits on federal funding for stem cell research raises the prospect that taxpayer money could be used for a much broader, much more controversial array of studies than many scientists, officials and activists anticipated.

Although the decision to allow expanded funding had been long expected, many thought Obama would limit federally funded scientists to working with cell lines derived from embryos destined to be discarded at infertility clinics. Instead, he left that key issue open.

The task of deciding what kinds of studies will be supported now falls to the National Institutes of Health, which finds itself confronting far more extensive questions than its officials were contemplating. It has 120 days to do the job.

Among other things, officials will have to decide whether to endorse studies on cells obtained from much more contentious sources, such as embryos created specifically for research or by means of cloning techniques.

"He left it wide open," said Thomas H. Murray, director of the Hastings Center, a bioethics think tank. "Now we are going to have to face a...