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Eye phenotypes were investigated in  Le-Cre Tg/−  ;  Pax6 fl/+   mice, which were expected to show tissue-specific reduction of Pax6 in surface ectoderm derivatives. To provide a better comparison with our previous studies of  Pax6 +/−   eye phenotypes, hemizygous  Le-Cre Tg/−   and heterozygous  Pax6 fl/+  mice were crossed onto the CBA/Ca genetic background. After the  Le-Cre  transgene had been backcrossed to CBA/Ca for seven generations, significant eye abnormalities occurred in some hemizygous  Le-Cre Tg/−  ;  Pax6 +/+   controls (without a floxed  Pax6 fl   allele) as well as experimental  Le-Cre Tg/−  ;  Pax6 fl/+   mice. However, no abnormalities were seen in  Le-Cre −/−  ;  Pax6 fl/+   or  Le-Cre −/−  ;  Pax6 +/+   controls (without the  Le-Cre  transgene). The severity and frequency of the eye abnormalities in  Le-Cre Tg/−  ;  Pax6 +/+   control mice diminished after backcrossing  Le-Cre Tg/−   mice to the original FVB/N strain for two generations, showing that the effect was reversible. This genetic background effect suggests that the eye abnormalities are a consequence of an interaction between the  Le-Cre  transgene and alleles of unknown modifier genes present in certain genetic backgrounds. The abnormalities were also ameliorated by introducing additional Pax6 gene copies on a CBA/Ca background, suggesting involvement of Pax6 depletion in  Le-Cre Tg/−  ;  Pax6 +/+   mice rather than direct action of Cre recombinase on cryptic pseudo- loxP  sites. One possibility is that expression of Cre recombinase from the  Pax6-Le  regulatory sequences in the  Le-Cre  transgene depletes cofactors required for endogenous  Pax6  gene expression. Our observation that eye abnormalities can occur in hemizygous  Le-Cre Tg/−  ;  Pax6 +/+   mice, in the absence of a floxed allele, demonstrates the importance of including all the relevant genetic controls in Cre- loxP  experiments.

Hemizygous Le-Cre Transgenic Mice Have Severe Eye Abnormalities on Some Genetic Backgrounds in the Absence of LoxP Sites