A number of viruses, particularly members of the poxvirus, herpesvirus and retrovirus families, have adapted to the vertebrate immune responses by capturing and modifying cellular genes which regulate the host immune system. Included among these host-derived virus genes are modified versions of receptors for cytokines or chemokines. Most of these receptor homologs, also called viroceptors, are either secreted glycoproteins or are located at the infected cell surface. Although these viroceptors can act in different ways, collectively they function by modifying the cytokine network to the advantage of the virus rather than the host.