and improving locomotor functional loss with no tumor formation for the observation period of more than 120 days (Tsuji et al., 2010). Besides, an in vivo study has shown that 3 rats have died out of 12 rats that received transplantation with DA neurons derived from protein based iPSCs (Rhee et al., 2011). The rats that died showed tumor formation after 8 weeks from grafting. Moreover, in a comparison between transplanted secondary neurospheres derived from iPSCs generated in 11 different ways and neurospheres from ESCs, the former showed a significant teratoma formation propensity most likely correlated with the persistence of undifferentiated cells (Miura et al., 2009). All this further halts the use of such a therapeutic tool in humans as much still needs to be optimized.
