The allelicNd-sandNd-s D mutations of the silkwormBombyx moriare mapped to the same locus as that of the fibroin light (L)-chain gene (Fib-L). The silkworm carrying the homozygousNd-sorNd-s D mutation secretes less than 0.3% of the normal level of fibroin and produces a thin cocoon (naked pupa). In this study, cDNA sequences of theNd-sandNd-s D L-chains were compared with the cDNA and genomic sequences of the L-chain of theB moriJ-139 strain, a normal-level producer of fibroin. The two mutant cDNA sequences are almost identical except for one base change in the coding region. The N-terminal half of the J-chain encoded by exons I to III is identical between the mutants and L-139, but the rest of the molecule is completely different. The C-terminal half of theNd-s D mutant L-chain is encoded by two exons, IV' and V', which are brought into proximity with the exon III by recombination between sequences in the third intron and in the far downstream region with concomitant loss of a region containing exons IV to VII. Sequences corresponding to exons IV' and V' are present about 10 kb downstream of the L-chain gene in the J-139 genome. Their homologous sequences have not been found in the DNA and protein databases. The chimeric L-chain molecule of about 27 kDa is present in posterior silk glands of Nd-s and Nd-s D strains without disulfide-bonding to the fibroin heavy (H-) chain, as revealed by Western blotting with the antibody specific to the C-terminal half of the mutant L-chain. f 2 f2Present address: M. Waga and S. Waga, Cold Spring Harbor Laboratory, Cold Spring Harbor, NY.Abbreviations used: PSG, posterior silk gland; MSG, middle silk gland; H-chain, heavy chain; L-chain, light chain; ER, endoplasmic reticulum; BiP, immunoglobulin heavy-chain binding protein.