essential to make progress in the development of treatments for human disease, and this research should continue.

Finding 2: Current scientific data indicate that there are important biological differences between adult and embryonic stem cells and among adult stem cells found in different types of tissue. The therapeutic implications of these biological differences are not clear, and additional scientific data are needed on all stem cell types. Adult stem cells from bone marrow have so far provided most of the examples of successful therapies for replacement of diseased or destroyed cells. Their potential for fully differentiating into other cell types (such as brain, nerve, and pancreas cells) is still poorly understood and remains to be clarified. In contrast, embryonic stem cells studied in animals clearly are capable of developing into multiple tissue types and capable of long-term self-renewal in culture, features that have not yet been demonstrated with many adult stem cells. The application of stem cell research to therapy for human disease will require much more knowledge about the biological properties of all types of stem cells. The best available scientific and medical evidence indicates that research on both embryonic and adult human stem cells will be needed. Moreover, research on embryonic stem cells will be important to inform research on adult stem cells, and vice versa.

Recommendation: Although stem cell research is on the cutting edge of biological science today, it is still in its infancy. Studies of both embryonic and adult human stem cells will be required to most efficiently advance the scientific and therapeutic potential of regenerative medicine. Research on both adult and embryonic human stem cells should be pursued.

Finding 3: Over time, all cell lines in tissue culture change, typically accumulating harmful genetic mutations. There is no reason to expect stem cell lines to behave differently. In addition, most existing stem cell lines have been cultured in the presence of nonhuman cells or serum that could lead to potential human health risks. Consequently, vigilant



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