“…There are several drawbacks for the genome integration system for example their infectivity is limited to dividing cells, thus restricting the range of cell types that can be reprogrammed; silencing occurs gradually during the course of iPSC induction, resulting in a lowered efficiency of conversion compared to nonsilencing viral methods and iPSCs made with retroviruses often maintain viral gene ex- Yamanaka, 2006 Blelloch et al, 2007;Yu et al, 2007;Brambrink et al, 2008Brambrink et al, 2008Stadtfeld et al, 2008b Free of genetic modification No genetic modification, still requires at least one factor to be transduced Okita et al, 2008Chang et al, 2009Soldner et al, 2009Stadtfeld et al, 2008cFusaki et al, 2009;Gonzalez et al, 2009Kaji et al, 2009Woltjen et al, 2009Bosnali and Edenhofer, 2008Huangfu et al, 2008aShi et al, 2008b;Ichida et al, 2009;Lyssiotis et al, 2009 pression thus limiting their utility (Maherali and Hochedlinger, 2008). Although iPSCs can be generated by constitutive lentiviruses, their poor silencing within pluripotent cells make them less suitable for direct reprogramming attempts (Blelloch et al, 2007;Yu et al, 2007;Brambrink et al, 2008). Drug-inducible lentiviruses have provided a more attractive approach, as they permit temporal control over factor expression.…”