Certain poxviruses express a member of the semaphore superfamily, which are a large group of cellular proteins that were first characterized in nervous tissue as regulators of guidance of sensory neurons, but members probably have immunomodulatory roles as well. The vaccinia virus semaphorin is encoded by the A39R gene (in strain Copenhagen) and is expressed as a secreted protein that binds to a cellular receptor called VESPR (virus-encoded semaphorin protein receptor), but the biological role of this protein has yet to be defined.