Parkinson’s disease results primarily from the loss of neurons deep in the middle part of the brain (the midbrain), in particular neurons that produce dopamine (referred to as “dopaminergic”). In this region of the midbrain there are actually two different groups of dopaminergic (DA) neurons, and only one of them, the neurons of the substantia nigra (SN) are highly susceptible to degeneration in patients with PD. There is a relative sparing of the second group of midbrain dopaminergic neurons, called the ventral tegmental area (VTA) dopaminergic neurons. These two groups of neurons reside close to each other in the brain and both make dopamine. They are virtually indistinguishable except for one major functional difference—they release dopamine, the transmitter that is lost in Parkinson’s patients, to their downstream neuronal targets in different ways. SN neurons deliver dopamine in small rapid squirts, like a sprinkler, whereas VTA neurons have a tap that provides a continuous stream of dopamine. A major therapeutic strategy for patients with PD is to make new DA neurons from human embryonic stem cells (hES). As stem cells have the potential to develop into any type of cell in the body, these considerations suggest that we should devise a way to produce SN neurons in the absence of VTA neurons from stem cells for use in transplantation. At present although we can produce dopaminergic neurons from hES cells, the scientific community cannot distinguish SN from VTA neurons in vitro due to lack of molecular markers or a bioassay, and we are therefore unable to identify culture conditions that favor the production of one over the other, In addition to releasing dopamine differently, SN and VTA neurons have axons that project to different regions of the striatum. It has been shown over the last decade that specific classes of guidance cues guide axons to their particular targets. One approach we have taken has been to investigate whether differences in axon guidance receptor expression and or responses to guidance cues in vitro might provide both markers and a bioassay that will distinguish SN from VTA neurons. We showed previously that VTA and SN neurons respond differentially to Netrin-1 and express different markers associated with the guidance cue family. Also, in this year using backlabeling, laser capture and microarray analysis of SN vs VTA neurons, we have identified a number of genes expressed in on or the other population. We now have a bioassay and markers that distinguish these two populations of neurons in vitro and in the coming year we plan to utilize this information to identify cultures conditions that favor the production of SN over VTA neurons, from hES cells.