 With few exceptions, and in stark contrast to avian and especially mammalian IFNs, they are present as single genes (multiple genes are however seen in polyploid fish genomes, possibly arising from whole-genome duplication).Unlike amniote IFN genes, piscine type I IFN genes contain introns, in similar positions as do their orthologs, certain interleukins.Despite this important difference, based on their 3-D structure these piscine IFNs have been assigned as Type I IFNs.