the results of experiments with stem cells in rodents are not useful. Those results have prompted new theories about the source and significance of regenerative capabilities in all cells, about the process of cellular differentiation, and about the role of the physiological environment in inducing cells of all kinds to express their different characteristics (Blau et al., 2001).

All somatic cells in an organism contain the same genetic information, but it is not yet known what causes parts of the genetic code to be expressed in some cells and different parts to be expressed in others. This raises important and interesting questions about the ability of a cell of one type to become another type. Emphasizing how little is understood about the process that controls a cell’s commitment to one course of action or another, Ihor Lemischka explained his findings that many genes found to be active in stem cells do not correspond to any known gene function ever described. A comparison of mouse and human HSCs shows that only about half of the genes expressed in the mouse HSCs correspond to genes expressed in human HSCs, so there are going to be differences as we move from experiments with mice to regenerative therapies in humans. Even the genetic programs that control the differentiation of human fetal liver stem cells and human HSCs, both of which evolve into the components of the blood, seem to be very different (Phillips et al., 2000). We need to understand much more about the differences between mouse and human stem cells if we are to harness their potential.