[10, 21]. Another very different integration free method is the direct delivery of reprogramming proteins into the cells [22]. Nevertheless, this approach is technically difficult and requires high amounts of special proteins. Again protein transfection also shows a low efficiency. A very promising tool is represented by transfected mRNA. It is a nonintegrating, virus free method which shows a comparably high efficiency [11]. Due to the fact that repeated transfections are necessary, this method is much more extensive and expensive compared to the viral methods and therefore not yet the common state of art for most applications.
