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Abstract: This paper reports the cloning and sequencing of interleukin (IL)-23 p19 subunit for the first time within a non-mammalian species, the zebrafish (Danio rerio), which was discovered using a synteny approach. In addition, amino acid sequences were for IL-23 p19 subunits were also predicted from the stickleback, Fugu and Tetraodon genomes and included in this investigation. The zebrafish IL-23 p19 cDNA consisted of a 66bp 5′ UTR, a 249bp 3′ UTR and a single open reading frame of 567bp giving a predicted 188 aa IL-23 p19 molecule. Multiple alignment of zebrafish IL-23 p19, with other known IL-23 p19 and IL-12 p35 amino acid sequences revealed areas of amino acid conservation, such as the presence of four predicted α-helixes, cysteines important for disulphide bond formation and the conservation of a tryptophan known to interact with the receptor. Amino acid homologies and phylogenetic analysis confirmed the relationship of the fish IL-23 p19 subunits with their mammalian homologues. All the teleost fish IL-23 p19 subunits were found to have 4 exons and 3 introns similar to that of human and mouse IL-23 p19 and a limited degree of synteny was found between the organisms for the regions containing the IL-23 p19 genes with only PAB-dependent poly(A)-specific ribonuclease subunit 2 (PAN2) and IL-23 p19 found in the same order on human chromosome 12 and all the fish genomes looked at. Lastly using real-time PCR, constitutive expression of IL-12 p40 and IL-23 p19 was observed in the kidney, liver, gut and muscle with IL-12 p40 expression higher than IL-23 p19. As soon as an hour after stimulation with LPS, there was an increase of IL-23 p19 in zebrafish leukocytes and an increase of IL-1β, IL-12 p40 and IL-23 p19 expression was found after infection of zebrafish for 1 or 6 days with Mycobaterium marinum strain E11. [Copyright &y& Elsevier]