Tissue regeneration may be common in certain species but is rare in mammals. Typical mammalian healing involves scar formation, and scar tissue is quite different from the original tissue that it replaces. Regeneration, on the other hand, involves formation of a blastema, a group of cells capable of rapid growth that recreates missing tissue. Regeneration is not completely absent in mammals: in 1996, Ellen Heber-Katz accidentally discovered that the MRL mouse strain was capable of tissue regeneration. Since then, this ability has remained something of a mystery. But now Heber-Katz (The Wistar Institute, Philadelphia, PA) and her colleagues have identified what makes MRL mice into ‘healers’: inactivation of the protein p21, a cell-cycle regulator.Lab Anim. (NY) 39, 127 (2010).