Hydra is able to regenerate any part of its body to rebuild an entire individual. The head organizer performs two opposite activities, one activating, which causes the head to differentiate, and the other inhibiting, which prevents the formation of supernumerary heads. Researchers have discovered the identity of the inhibitor, called Sp5, and deciphered the dialogue between these two antagonistic activities, which helps maintain a single-headed adult body and organize an appropriate regenerative response.