Stem cells are defined by their capacity for self-renewal and ability to differentiate into a variety of somatic cell types. Cellular programs regarding proliferation, potency, and cell fate determination can be mediated by signal transduction events that modulate transcription factor expression and/or activation. Transcription factors promote the differentiation of embryonic stem (ES) cells derived from the inner cell mass of the blastocyst into stem or progenitor cells of all three vertebrate germ layers: ectoderm, mesoderm, and endoderm. In addition, the expression of specific transcription factors is sufficient to reprogram somatic cells back to a pluripotent state. Induced pluripotent stem (iPS) cells, like ES cells, give rise to adult stem cells such as neural and epithelial stem cells. Adult stem cells function to replenish cells of the tissue in which they are found.