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What makes you human?
A scientist looking through a microscope.You are a developmental biologist who explores how single, undifferentiated cells develop into complex organisms. For most of your career you have worked on stem cells, particularly embryonic stem cells—a class of cells remarkable (and unique) for their ability to develop into many different kinds of cells and to renew themselves perhaps indefinitely. Your work has led to a better understanding of these regenerative properties, and clinicians are now exploring the very promising use of these cells to treat a huge range of medical problems, from spinal cord injuries to diabetes. Now many of your colleagues want to inject human embryonic stem (ES) cells into mouse embryos in order to create mixed-species embryos and to test the cells' clinical usefulness. As you were among the first to isolate ES cells, your colleagues recognize your expertise and stature in the field, and want your support.
Should you endorse research that creates mixed-species embryos that are part human?
Clicking on "yes" or "no" will move you to the next page. You will have a chance to reconsider your answer when you get to the end of the case and have explored some of the ramifications of your decision, but you cannot click back!
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Context

Stem cells could potentially benefit millions. For example, if, as research indicates, stem cells can help treat Parkinson's and diabetes, treatment for those two diseases alone could potentially help 17 million Americans. Because stem cell research is still in its early stages, there are potentially many other stem cell uses we are not presently aware of. The Coalition for the Advancement of Medical Research estimates that up to 100 million Americans could benefit from stem cell research.

Look Deeper
To test whether human embryonic stem cells could actually become other cells, researchers would inject human embryonic stem cells into mouse embryos, then re-implant the embryos into female mice and allow them to develop. At various stages of development the embryos would be dissected to see if they contained human cells.
What makes you human?
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Did you know?
Stem cells have the ability to develop into many kinds of cells found in the human body. There are three kinds of stem cells: embryonic stem (ES) cells, which are derived from early embryonic cell masses; embryonic germ (EG) cells, which come from aborted human embryos/fetuses and have developed into reproductive cells (sperm and egg); and adult stem cells, which can be found among the many cell types in a particular organ or tissue in the body. Embryonic stem cells are considered pluripotent, which means that they can give rise to almost any human cell type. Embryonic germ cells, while not pluripotent, are proving more able to differentiate into different cell types than originally thought. Adult stem cells can be multipotent cells (able to grow into many different kinds of cells within a specific type of tissue or organ) or progenitor cells (immature cells with a very limited ability to differentiate), and recent research suggests that some adult stem cells may be pluripotent. However, most specialists in the field contend that adult stem cells do not share the range of remarkable properties that ES cells have.

Scientists have been aware of stem cells from at least the early 1900s, and fetal nerve cells have been used for transplants for almost 15 years in Parkinson's patients. In 1998 a scientist at the University of Wisconsin isolated and grew embryonic stem cells for the first time. Experts on embryonic stem cells expect that clinical trials for ES cells are anywhere from 5 to 10 years away.