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Fig. 1 The genetic locus of llb encodes the zebrafish orthologue of isl1. A, B: Morphology of the wild-type (A) and llb embryos (B) at 28 hpf. C: Genetic map of the llb locus. The llb locus was genetically mapped between the simple sequence-length polymorphism (SSLP) markers L546723 (0.21 cM) to G47922 (0.21 cM) on chromosome 5. D: Splicing defect of Isl1 caused by the llb mutation. Bracket, extra 10-bp insertion into the isl1 cDNA from the llb homozygous embryos; shaded letters, original splicing sites; red arrowheads, nucleotide where an A-to-G substitution has occurred; red letters, newly formed splicing acceptor site; green letters, wrong amino acid sequence. E: Amino acid sequence of Isl1 in the wild-type and llb embryos. Red arrowheads, exon-intron boundaries; green letters, wrong amino acids that were generated by the llb mutation. 182T is replaced with a stop codon (red arrow). F: Structure of Isl1. The llb mutation caused deletion of the C-terminal region of Isl1, which includes the homeodomain. G: The RT-PCR product containing the boundary between exons 3 and 4 of Isl. Black arrows, primer sites; red arrow, 10-base longer fragment that was only observed in the llb embryos.