incomplete suppression of the four transgenes after differentiation, the persistence of undifferentiated cells, and the survival of the transplanted cells in vivo (Okita et al., 2007). The known risks so far that were obtained mostly from animal models include tumor formation, unwanted immune responses and the transmission of certain adventitious agents (Herberts et al., 2011; Okano et al., 2013). Unfortunately, an estimated 20% of mice that received iPSCs were found to develop tumors (Abdullah et al., 2012). Furthermore, it has been hypothesized that the sustained expression of the transgenes might have the ability to change the expression of certain oncogenes or tumor suppressor genes thus altering the tumorigenic potential of the cells. The c-Myc is a risk factor by itself as it is upregulated in naturally occurring tumors.
