As a cautionary note, the absence of midbrain character is a critical limitation for clinical application, since only “authentic” human midbrain dopamine neurons are able to restore function in animal models of Parkinson’s disease (Kriks et al., 2011; Roy et al., 2006; Yang et al., 2008). Therefore, yet another group very recently attempted to generate iN cells that are more reminiscent of midbrain dopamine neurons (Kim et al., 2011). This time transcription factor combinations were screened to induce EGFP fluorescence in Pitx3:EGFP knock-in fibroblasts, a locus highly specific for midbrain dopamine neurons. Surprisingly, EGFP+ cells were readily detected with a combination of two factors Ascl1 and Pitx3. Complementation with another four factors (Nurr1, Lmx1a, Foxa2, and En1) as well as the patterning factors Shh and FGF8 further enhanced the number of EGFP+ cells. The EGFP+ cells also expressed the generic dopamine neurons markers TH, DAT, AADC, and VMAT2 and were able to release dopamine. However, when tested in vivo, the cells only partially restored dopamine function and when a series of midbrain markers were analyzed both the 2-factor and the
