Essentially, switching off the p21 gene allows adult cells to behave like pluripotent stem cells, reorienting themselves into whatever kind of tissue they need to be. But naturally there is a give-and-take; p21 is closely intertwined with another gene, p53, a cell-division regulator that, if allowed to run amok, can lead to many types of cancers. The p21 gene acts as a safety valve for p53, stopping cell division in the case of DNA damage. So switching off p21 can allow cells to engage in regenerative healing, but the risks of doing so include rampant cell division (read: cancer).