Schematic structures of IgH loci of the Atlantic salmon and other vertebrates (not to scale). The Atlantic salmon duplicated IgH loci, IGH-A (670 kb) and IGH-B (710 kb), were completely sequenced in this study. The diagram of the IgH locus of zebrafish (175 kb), stickleback (175 kb) and human (1, 250 kb) are modified from references, [21, 28, 29] and , respectively. To date, only 3' end regions of approximately 100 kb in the rainbow trout  and 260 kb in the catfish IgH locus have been sequenced. In the catfish locus, linkage among IGH1, IGH2 and IGH3 was established by restriction mapping and Southern blot analyses [26, 82]. The complete sequences of the catfish IGH2, IGH3 and the upstream of IGH1 VH region have not yet been reported. Therefore, these regions are shown in gray boxes. The continuous regions are indicated by dotted lines, while the gap regions are indicated by double slashes (//). Dashed-line boxes indicate the CH pseudogenes (ψ). Transcription directions are shown by arrowhead.