Human embryonic stem cells (hESCs) are derived from the inner cell mass of blastocyst-stage embryos that have been fertilized in vitro. In mammals, all embryonic cells remain totipotent and have the capacity to form an entire organism– through the morula stage (about day four after fertilization). Beyond the morula stage, cells begin to specialize. Stem cell populations, other than those derived from very early embryonic cells, can thus be classified as either pluripotent or multipotent. The hESCs used in research are typically pluripotent and are able to differentiate into derivatives of any of the three germ layers: ectoderm, mesoderm and endoderm. Adult stem cells, by contrast, are multipotent and thus are only able to differentiate into a specific subset of cell types. The pluripotency of hESCs leads to implications in the field of regenerative medicine and transplantation therapies.