Stem cells are a diverse class of cells that share the ability to self-renew and to differentiate into multiple cell types. There are two main types of stem cells. Embryonic stem cells (ESCs), which are found only in the embryo, can form any cell type present in the body. Adult stem cells (ASCs), in contrast, are already committed to a particular lineage but may be capable of forming multiple cells types within their restricted lineage. ASCs can be found in various tissues throughout the adult body. Among the types of ASCs that have been extensively characterized are neural stem cells (NSCs), mesenchymal stem cells (MSCs) and hematopoietic stem cell (HSCs).