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In this paper we describe the gene organisation of sea bass Dicentrarchus labrax interleukin-1β (IL-1β) and the presence of incompletely spliced forms of this gene. Interestingly, the sea bass IL-1β gene comprises five exons and four introns, thus being remarkably different from other known teleost and mammalian genes. The sizes of the introns were typically much shorter than in mammals and this feature, together with the loss of two introns, gave a much smaller gene than in mammals (2.7 vs 6.5–7.0 kb). The 5′-end exon–intron organisation seems quite different in the known IL-1β genes with three exons in human and carp, two in trout and only one in sea bass. The highest percentage of exon identity and similarity is between sea bass exon 4, trout exon 5, carp, mouse, chicken, pig and human exon 6. RT-PCR analysis revealed the presence of two incompletely spliced transcripts. Southern blot analysis suggests the presence of only one copy of IL-1β gene in sea bass genome.