This diagram illustrates the range of stem cell potency, based upon the cells’ state of differentiation. The more potent a cell, the less differentiated it is. The least differentiated state, the fertilized egg, is considered totipotent. Embryonic stem cells are from a slightly later stage of development and are considered pluripotent. Still less potent are committed precursors, which are destined to assume a fate within a particular tissue type. The most differentiated cells are specialized cells, which have assumed only one fate from the more than 260 different types of specialized adult cells, such as white blood cells, red blood cells, muscle cells, or skin fibroblasts. The diagram also illustrates deprogramming of specialized cells using “stemness” genes (Oct3/4, Sox2, and Klf4) to take them back to a pluripotent state, known as induced pluripotent stem cells.