The focal nature of neuronal loss suggests that cell replacement therapy may be suitable for Parkinson's disease. Indeed, transplanted fetal midbrain cells survive in some Parkinson's patients for over a decade, which contributes to sustained symptom relief. However, the outcome of the transplant therapy trials is often variable, mainly due to the poorly characterized fetal tissues from random donors. The objective of this research is to generate and standardize transplantable dopamine neurons from human embryonic stem (ES) cells, cells that can be replicated almost indefinitely while maintaining the potential to generate all cell types in our body. We have established a culture system to induce human ES cells to give rise to precursors of brain and spinal cord cells. We will create a culture system that allows the neural precursors to generate dopamine neurons using cocktails of growth factors based on what we have learned from developmental biology. In this dopamine neuron induction culture, we will then identify an immature progenitor stage by inserting into cells a piece of fluorescent-tagged DNA that will be switched on only when the cells turn into dopamine neurons. This way, we will optimize the culture system to generate maximal number of transplantable dopamine neurons using non-genetic approaches. The function of the in vitro generated dopamine neurons will be evaluated after transplantation into a rat PD model that is established in our lab. We hope that this proposed study will form a basis for future preclinical and clinical trials using human ES cells as an unlimited and standard source for dopamine neurons.