The past 5 years have witnessed an explosion of interest in using adult-derived stem cells for cell and gene therapy. This has been driven by a number of findings, in particular, the possibility that some adult stem cells can differentiate into non-autologous cell types, and also the discovery of multipotential stem cells in adult bone marrow. These discoveries suggested a quasi-alchemical nature of cells derived from adult organs, thus raising new and exciting therapeutic possibilities. Recent data, however, argue against the whole idea of stem cell 'plasticity', and bring into question the therapeutic strategies based upon this concept. Here, we will review the current state of knowledge in the field and discuss some of the clinical implications.