Type I interferons are proteins with antiviral, antitumour and immunomodulator activities. We solved the crystal structure of recombinant murine interferon-β (reMuIFN-β) using an X-ray crystallographic technique. Here, we describe a structure-function relationship of Type I interferons based on the crystal structure of reMuIFN-β. Furthermore, we discuss the evolution of interferons and helical cytokines.