At the Duke University Medical Center, biologists have been working with zebrafish, a common aquarium fish with unusual properties, namely, they can regenerate damaged limbs and organs, including fully functional eyes and hearts. They can re-grow an entire fin in approximately two weeks' time assuming that the fish is otherwise healthy. As it turns out, very small pieces of RNA (ribonucleic acid, which is involved in the synthesis of proteins, as well as controlling the state of certain genes) control whether or not the regeneration mechanism is active or not. If a particular micro-RNA strand, designated miR-133, has a low …