The hammerhead ribozyme is a RNA molecule motif that catalyzes reversible cleavage and joining reactions at a specific site within an RNA molecule. It serves as a model system for research on the structure and properties of RNA, and is used for targeted RNA cleavage experiments, some with proposed therapeutic applications. Named for the resemblance of early secondary structure diagrams to a hammerhead shark, hammerhead ribozymes RNAs were originally discovered in two classes of plant virus-like RNAs: satellite RNAs and viroids. They have subsequently been found to be widely dispersed within many forms of life.