While criteria for self-renewal and multipotentiality can be met with varying degrees of confidence on the basis of recent in vitro and in vivo experiments, a consensus definition of stem cells remains illusive. It has been demonstrated that adult CNS cells can divide and be induced to differentiate both in vivo and in vitro. These observations have important basic and applied implications; however, the true identifies of these cells and factors, as well as the mechanisms that control their fate, are unknown.