of more widespread cortical neuronal loss. Clinical trials of cell transplantation have already been performed for each of these prototypic neurodegenerative conditions (reviewed in (Barker et al., 2015; Barker et al., 2013; Benraiss and Goldman, 2011; Lindvall and Bjorklund, 2011)). But these trials used fetal tissues dissected from the regions of interest, which thus included all cell types in the tissue, and not just the specific populations of nigrostriatal and striatal medium spiny neurons respectively lost in PD and HD; in each of these cases, the target cell types typically comprised but a fraction of the cells delivered. Perhaps as a result, fetal tissue grafts into Parkinson’s patients have yielded variable results, with both clear successes and failures, and a disturbingly high incidence of refractory dyskinesias, in which uncontrollable movements can negate the functional gains otherwise afforded by the grafted cells (Barker et al., 2015). Similarly, fetal striatal grafts into patients with Huntington Disease have yielded mixed results, with little evidence of significant or durable functional improvement (Cicchetti et al., 2009). Thus, the specific generation of midbrain dopaminergic neurons and medium spiny neurons from human embryonic stem cells and iPSCs (An et al., 2012; Carri et al., 2013; HDConsortium, 2012;
