Pluripotent stem cells are the most versatile kind of stem cells, having the capacity to differentiate into any of the somatic cell types that arise from the three primary germ layers (endoderm, ectoderm, and mesoderm) during the development of an embryo. In addition to their pluripotent nature, these cells also possess unlimited self-renewal abilities in contrast to differentiated cells in the body in tissues such as skin, gut, or muscle that can divide only a finite number of times. Because of these two properties, they hold great promise both in research to understand development and human disease and in the clinic where they could be used to treat a wide variety of disorders that use tissue engineering or cell transplantation therapy. Until recently, the only ...