Embryonic stem (ES) cells display a prodigious ability to differentiate into diverse cell types and are seen as a promising renewable source of specialized cells for use in regenerative medicine. Since the use of human stem cells raises ethical concerns and is tightly regulated, animal models provide a useful tool to further explore the potential of this technology. In the September 21 Nature Biotechnology, Tiziano Barberi and colleagues at the Memorial Sloan-Kettering Cancer Center demonstrate that transplantation of dopaminergic neurons derived from cloned mouse ES cells can significantly improve the condition of parkinsonian mice (Nature Biotechnology, DOI:10.1038/nbt870, September 21, 2003).