 So far, IFN-ζ is only found in mice, while a structural homolog, IFN-δ is found in a diverse array of non-primate and non-rodent placental mammals.Most but not all placental mammals encode functional IFN-ε and IFN-κ genes.IFN-ω, although having only one functional form described to date ("IFNW1"), has several pseudogenes