Stem cells are functional units in both development and tissue homeostasis and can be found in a variety of embryonic and adult mammalian tissues. These cells are thought to arise from totipotent embryonic stem (ES) cells of the inner cell mass of the blastocyst from which distinct groups of precursors segregate into the three main germ layers (ectoderm, mesoderm, and endoderm) at around the time of gastrulation. Gradually, these cells will mature into fate-restricted organ- and tissue-specific somatic stem cells (SCs) (1), which are responsible for the growth of tissues during development. The number of SCs declines when the tissues approach maturity and remains rather constant throughout life.