Two failed attempts to transplant adult stem cells into the hearts of laboratory mice are casting doubt on the value and safety of clinical trials testing a similar approach to repair the hearts of humans.

In two separate papers released Sunday in the online version of the journal Nature, researchers at Stanford University and another team of scientists from Indiana University and the University of Washington describe how efforts to use adult stem cells from a mouse to regenerate heart muscle in other mice simply did not work.

The setback raises questions about similar experiments among human subjects in Brazil, Germany, Britain and China.

The Bush administration has sharply restricted federal funding for research on human embryonic stem cells, which are derived from living human embryos only a few days old. Opponents of such research contend that the necessary destruction of these embryos destroys a human being. They favor using adult stem cells, such as those found in bone marrow. These stem cells can differentiate into blood cells and can make other human organs and tissues.

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