IgM H chain locus in shark. Representation of functional H chain locus of Groups 1–5. Each consists of leader (L) and the gene segments VH, D1, D2, and JH in close proximity, the intersegmental regions each being ~400 bp. The triangles represent recombination signal sequences (RSS), with the 23 bp spacer (△) or 12 bp spacer (▲). The C region exons are located ~7 kb downstream in the G2 subfamily, and the exons are variously 0.5 to 3 kb apart. Universal PCR primer are shown above, as arrows; for instance, primer pair GR1–5/JH5 will amplify any nurse shark VH genes from G1–5.