Stem cells are undifferentiated, multipotential somatic cells from which other, specialized cells arise by differentiation. Stem cells are cells which have the ability to divide infinitely, and to develop into the huge variety of specialized cells required for the creation of an organism. Most cells have a specific function (e.g., liver cells, skin cells, brain cells, etc.) and once they have taken on this function, in a process called differentiation, they cannot be adapted for any other function. Stem cells, however, have not gone through the differentiation process.

By isolating stem cells in a laboratory, scientists theoretically could grow new heart cells to repair damage from heart attacks, new liver cells to treat hepatitis, or new red blood cells for cancer patients. As of 2001 the methods by which customize stem cells with respect to function is as yet unknown.

A number of different types of...