However, a recent in-depth study of IL-4/13 paralogs in trout indicated that whereas IL-4/13A is robustly expressed in various organs, including the gill, the IL-4/13B1 and IL-4/13B2 (paralogs apparently derived from an additional genome duplication event specific to salmonids) genes are more responsive to viral or parasitic infection in vivo (47). Here, IL4 is linked to parasitic infectious disease.