As of 2011, we have at least 93 government-approved embryonic stem cell lines (Kim 2011 ). Until we are able to test stem cells from various sources (embryonic, fetal, adult, and iPS) side by side in the laboratory, in a variety of experimental paradigms, the answer as to whether adult, embryonic stem cells, or iPS cells could serve therapeutic purposes and which source would be most efficacious, will remain unresolved. As stem cell technologies continue to evolve, it will be critical to follow how the ethical, legal, and social concerns associated with these technologies are addressed. You will find a discussion of these profoundly important issues in Module 2 and Supplement 1.

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