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In mammals,interferon regulatory factor 4(IRF4)plays an important role in the process of development and differentiation of B cells,T cells and dendritic cells.It can regulate immune pathway through IRF5,MyD88,IL21,PGC1,and NOD2.In the present study,we investigated the expression pattern of IRF4 paralogues and these related genes for the first time in teleosts.The results showed that these genes were all expressed predominantly in known immune tissues while IRF5 was also relatively highly expressed in muscle.IRF4b,IL21,MyD88,IRF5 and NOD2 showed maternal expression in the oocyte and the higher expression of IRF4a,Mx and PGC1 before hatching might be involved in the embryonic innate defense system.LPS significantly down-regulated zebrafish IRF4a and IRF4b expression at 4 and 24 h while it up-regulated the expression of Mx,IL21,NOD2 and PGC1 at some time points.LPS and poly I∶C did not affect IRF5 transcript level.However,poly I∶C up-regulated the expression of Mx,IL21,NOD2 and PGC1 and down-regulated the MyD88 transcript at most time points examined.As mammalian IRF4,teleost IRF4a was refractory to poly I∶C treatment while IRF4b could be induced by poly I∶C.After infection with grass carp reovirus or spring viremia of carp virus,the expression of IRF4a and IRF4b was totally different.The IRF4a was increased while IRF4b was decreased.Inducible expression pattern and phylogenetic tree imply that tetrapod IRF4 may come from teleost IRF4a.Furthermore,IRF4a and IRF4b displayed a distinguishing expression pattern in vivo and in vitro,suggesting that IRF4 paralogues might play different roles in immune system.