Zebrafish can regenerate amputated fins back to their original size. It is now shown that gradients in extracellular signal-regulated kinase activity act as rulers during fin bone regeneration. ↧ Decaying and expanding Erk gradients process memory of skeletal siz... Nature Physics - Zebrafish can regenerate amputated fins back to their original size. It is now shown that gradients in extracellular signal-regulated kinase activity act as rulers during fin bone...