For culturing keratinocytes, special low-calcium medium formulations are available to prevent the cells from getting senescent too early [70]. This is a benefit because on one hand most other cells do not proliferate well in the low-calcium medium and, on the other hand, after starting the reprogramming, a new medium with normal calcium amounts is used whereby all uninfected keratinocytes reduce or stop proliferation. Only infected cells continue to proliferate, a prerequisite to reprogram into stem cells (Figure 4).
