Stem cells are undifferentiated cells, that is, those that do not perform a specialized function but are capable of changing into specialized cells. All human beings start out as a single cell, and stem cells are common in the developing fetus. However, as a human being grows and develops, cells become more differentiated and specialized. Although adults do have stem cells, they are not common in the body.

Medical science is now studying stem cells as a possible treatment for diseases where the body is unable to create or replace specialized cells. In theory, if stem cells are placed in the appropriate place in the body, they should be able to differentiate into the appropriate type of cell. If this is the case, many conditions from diabetes mellitus to degenerative nerve diseases could be successfully treated.

Currently, most stem cells are harvested from aborted fetuses and are grown in cultures. The source of stem cells has caused a great deal of controversy over the research and during the administration of George W. Bush, the federal government did not fund stem cell research based on cells from aborted fetuses. However, several private foundations, such as the Howard Hughes Medical Institute, do fund such research.

In Not Cancer, House deals with patients who die when stem cells become cancerous, take the place of healthy cells, but do not differentiate into the appropriate cell.

In Small Sacrifices, the patient initially refuses stem cell treatment for his multiple sclerosis.

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