When the human body becomes infected with new influenza viruses, the immune system rapidly activates an inborn protective mechanism to inhibit the intruding pathogen. A protein known as Mx plays an important role in this process, keeping the spread of viruses in check. Exactly how Mx accomplishes this task was previously unknown. Now virologists and structural biologists have unraveled the structure of the Mx protein and are able to explain how it develops its anti-viral effect.