The classification of mammalian cytokines with antiviral activity, is well documented as being either Type I Interferon (IFNI) or Type II Interferon (IFNII). This is not the case for avian IFNs however, even though IFN was first detected in the chicken. A growing number of functional studies indicate similarities between chicken IFN and mammalian IFNI, revealing the conservation of cysteine residues and the existence of around ten closely related intron-less IFN alpha genes. Further research has shown that chIFN-alpha genes are strongly expressed following viral infection of monocyte-derived macrophage and embryo fibroblasts and that recombinant chicken IFN acts as a potent antiviral agent.