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Vitellogenins (Vtg) are the major yolk proteins in most oviparous organisms. They are encoded by a small number of genes – between one and four depending on the species. Characterization of the Vtg region in the genome of the rainbow trout reveals unusual features, however, in that this locus contains twenty complete genes and ten pseudogenes per haploid genome. The Vtg genes differ from each other by insertion, deletion and rearrangement events, although, at the sequence level, they show a high degree of similarity. Fluorescent in situ hybridization (FISH), pulsed-field gel electrophoresis (PFGE) and Southern analysis indicate that all gene copies are contained in a single 1500-kb region, and that most of the genes form tandem arrays separated by a conserved 4.5-kb intergenic region. The presence of large reiterated fragments indicates that this region has been subjected to several …