The mammalian liver possesses a unique capacity for regeneration. However, this regenerative potential declines with age due to unknown mechanisms. In this issue of Genes & Development, Ritschka and colleagues (pp. 489–494). compare liver regeneration upon partial hepatectomy in young and adult mice. Partial hepatectomy causes a transient increase in p21 in a subpopulation of hepatocytes that persists in adult mice. Remarkably, treatment with the BCL-2 family inhibitor ABT-737 blunts p21 expression, enhancing liver regeneration.