Occurrence of the Heterozygosity at Es-1 Locus in a Sub-strain of the NZB Mice

In the course of inspection of the biochemical marker genes in inbred strains of mice maintained in our laboratory, a female mouse of the NZB strain was found to be heterozygous for the Es-1 locus. Namely, it was Es-1a/Es-1b type. After this finding, many heterozygous mice were found among her sisters and the descendants. However, these heterozygotes (Es-1a/Es-1b) showed no heterozygosity for other 11 characters, i.e., the 6 biochemical markers (Hbb, Trf, Es-2, Id-1, Mod-1, Gpd-1) and the 5 coat colour markers (A, B, C, D, AND S) were idential as those previously described. It was, moreover, observed that they possessed the immunological characteristics typical of the NZB mice. Therefore, it could be concluded that the heterozygosity had been originated from a single mutation at the Es-1 locus, i.e., from Es-1a to Es-1b or vice versa. With regard to the alleles at the Es-1 locus, an investigation was carried out in two sub-strains of the NZB mice having different breeding history and the followings were clarified. One substrain imported from Karolinska Institute, Sweden, had been fixed with the Es-1a allele and the other imported from England was found to be Es-1b/Es-1b type. The NZB mice which displayed the heterozygosity had been derieved from the Karolinska sub-strain. Importance of biochemical marker genes for inspection of proper maintenance of inbred strains has been discussed.