Eo: a history of a mutation.

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Abstract
Eighteen mouse t haplotype‐carrying strains were found not to express cell‐surface E molecules controlled by class II genes of the H‐2 complex (= Eo strains). Northern and Southern blot analysis of these and other, non‐t strains that also fail to express the E molecule, has revealed two kinds of defect. Three strains (CRO437, tw2, and presumably to) were found to transcribe the E alpha gene, but they were not able to convert the message into a functional protein. All other Eo strains fail to transcribe the E alpha gene because of a deletion encompassing the promoter region, the RNA initiation site, and the first exon. The length of the deletion is approximately 650 +/‐ 50 bp. These two defects closely resemble those found previously in standard inbred strains carrying the H‐2f, H‐2q (failure of E mRNA to be expressed functionally), H‐2b, and H‐2s (deletion of a part of the E alpha gene) haplotypes. In particular, the location and length of the E alpha deletion appear to be the same in the strains carrying this mutation. The E alpha deletion is in linkage disequilibrium with certain alleles at other H‐2 loci in some of the strains. These observations, combined with the growing evidence that H‐2 haplotypes associated with t chromosomes derive from a single ancestral haplotype, suggest that the E alpha deletion is an old mutation and that it has been disseminated in mouse populations by the t chromosomes.