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Genomic Southern blotting analyses of Bombyx mori and B. mandarina were performed with the 5′-exons or 5′-flanking sequences of two members of genes A/B.L11 and A/B.L12. They represent multigene families encoding chorion proteins that are expressed in the middle period of choriogenesis. Many bands cross-hybridized with the probes in B. mandarina and in different strains and races of B. mori, supporting the idea that the middle chorion multigene families have remained essentially the same in the two species. Both the exon and the 5′-flanking probes exhibited similar patterns within the European stocks of B. mori, indicating that the examined DNA regions have been conserved. Patterns revealed in a Japanese and a Chinese strain showed distinct differences in band number and fragment size, indicating that the chorion gene families are not fixed within the domesticated silkworm. A greater number of fragments hybridized with the exon probes in B. mandarina than in B. mori; this may have relevance to the previous finding that B. mandarina has a large number of proteins with high cysteine content. The banding patterns for both the exon and 5′-flanking probes were markedly different between B. mori and B. mandarina, indicating that major evolutionary differences exist between the two species.