vivo in animal models for Parkinson’s disease. To address this question, Kim and colleagues subsequently used tail tip fibroblasts from Pitx3-GFP knock-in mice and screened for factors that induce GFP expression[32]. Pitx3 is a member of the RIEG/PITX homeobox family and is important for the early development and survival of midbrain dopaminergic neurons [33–35]. Using those knock-in GFP reporter fibroblasts, the authors managed to narrow 11 transcription factors to only two: Ascl1 and Pitx3. But from gene expression analyses, the two factor iDaN cells only partially upregulated other midbrain dopaminergic markers. In search for ways to improve the midbrain character of the induced cells, the authors added more transcription factors (Lmx1a, Nurr1, Foxa2, and En1) and midbrain patterning factors (Sonic hedgehog, SHH, and fibroblast growth factor 8, FGF8). These combinations yielded about 9.1% Pitx3-GFP-positive cells by day 18 post infection. Not only did these iDaN cells exhibit the expected marker expressions (TH, DAT, AADC and VMAT), they also released dopamine in vitro and in vivo when they were transplanted into the 6-hydroxydopamine lesioned striatum model of Parkinson’s disease and showed electrophysiological properties similar to that of primary dopaminergic neurons. While this optimized protocol yielded iDaN cells that are able to
