Stem cells can be classified into three broad categories, based on their ability to differentiate. Toti potent stem cells are found only in early embryos. Each cell can form a complete organism (e.g., identical twins). Pluri potent stem cells exist in the undifferentiated inner cell mass of the blastocyst and can form any of the over 200 different cell types found in the body. Multi potent stem cells are derived from fetal tissue, cord blood and adult stem cells. Although their ability to differentiate is more limited than pluri potent stem cells, they already have a track record of success in cell-based therapies.