The promise of stem cells in Parkinson disease.

Neurotransplantation as a treatment for Parkinson disease reached the stage of human trials over 15 years ago, but the field, which is still in its infancy, has encountered a number of roadblocks since then, both political and scientific. With hope that stem cells may be used as a new source of dopaminergic neurons to replace the degenerating nerve cells in Parkinson disease looming, it is critical that we learn from the past as we work toward achieving new milestones aimed at making this new therapeutic strategy a reality. One of those milestones, which is an important translational step in the development of stem cell technology and the subject of a report in this issue of the JCI, involves transplanting new dopaminergic cell lines to a primate model of Parkinson disease.