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The mutation in the rd8 mouse has been identified as a single base deletion of a C (G on forward strand) at coding nucleotide 3481 in the gene. This deletion causes a frame shift and a premature stop codon that truncates the transmembrane and cytoplasmic domain of the protein after amino acid 1207. This mutation has been found to be present in all sublines of C57BL/6N and in C57BL/6ByJ, but not in any C57BL/6J subline. It occurred sometime between transfer of mice from JAX to NIH, in 1951, and from NIH to Donald Bailey, in 1961.

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This allele encodes a high affinity, heat labile, 104 kDa receptor containing 848 amino acids. Sequencing studies of cDNA from C57BL/6J congenic mice homozygous for this allele identified nucleotide substitutions in the ORF that would cause 5 amino acid differences between the C57BL/6J and BALB/cBy peptides, and 2 amino acid differences between the BALB/cBy and DBA/2J peptides. A T to C transition in exon 11 replaces the opal termination codon in the C57BL/6J allele with an arginine codon in the BALB/cBy allele. This change would extend translation of the BALB/cBy mRNA by 43 amino acids, accounting for the larger size of the peptide produced by this allele (104 kDa, vs 95 kDa for the C57BL/6J allele).

C57BL/6 carries the responsive Ahr^b allele; DBA/2 carries nonresponsive Ahr^d. Heterozygotes (Ahr^b/Ahr^d) are responsive (J:5282). Later work identified a second (J:8895) and later a third (J:22144) allele conferring response. Thus the allele in C57, C58, and MA/My strains is now Ahr^b-1; Ahr^b-2 is carried by BALB/cBy, A, and C3H; and Ahr^b-3 by Mus spretus, M. caroli, and MOLF/Ei. The nonresponsive strains AKR, DBA/2, and 129 carry Ahrd (J:22144). Nucleotide and amino acid sequence differences between Ahr^b-1 and Ahr^d have been determined (J:17460).

Strain of origin - this allele was found in BALB/cByJ, A/J, C3H/HeJ, CBA strains

An insertion of a 491-bp mouse leukemia virus long terminal repeat (MLV-LTR) sequence into intron 10 results in aberrant alternative splicing of the transcript with exclusion from most mature mutant mRNA species of exon 8IIIb, which encodes the domain required for specific binding to FGF ligands in developing prostate and seminal vesicle mesenchyme.

This recessive autosomal mutation apparently occurred early in the inbreeding of the Essex CXB recombinant inbred lines, as the abnormality was not present in either of the parental strains, C57BL/6ByEss or BALB/cByEss, but is present in both CXBI/ByJax and CXBI/ByLac J:9441.