According to this article scientists have identified a gene that prevents regeneration. It is thought that by switching off this gene may one day enable mammals to regrow lost limbs. Trials have been conducted on mice where the p21 gene was switched off. The mice lacking the gene were then able to gain the ability to regenerate lost and damaged tissues. They do this by forming a blastema, a structure which is associated with rapid cell growth and causes the cells to behave more like embryonic stem cells than normal mammal cells. Scientists hope that they might one day be able to accelerate healing in humans by switching off this gene temporarily.