While microglia may contribute to the pathogenesis of neurodegeneration, such as in multiple sclerosis [33], Alzheimer's disease [34], and acquired immunodeficiency syndrome-associated dementia [35], their role in PD [36] is most interesting in the context of these results, since GDNF has been used in clinical trials to protect degenerating dopamine neurons as well as promote regeneration of the nigrostriatal dopamine system [37, 38]. The gene discussed is GDNF; the disease is Parkinson disease.