Homozygous mice die between embryonic day (E)13.5 and birth with defects of heart, kidney, gonads, and multiple other organs. Heterozygous (Wt1GFPCre/+) mice are viable and fertile. The Wt1GFPCre "knock-in" allele both abolishes Wt1 gene function and expresses an enhanced green fluorescent protein-Cre recombinase fusion protein (EGFPCre) from the Wt1 promoter/enhancer elements. In heart from heterozygous mice, EGFPCre expression is directed to proepicardium and epicardium from E9.5 to E15.5, and is not found in the myocardium. When bred to mice containing loxP-flanked sequences, the resulting offspring will have Cre-mediated deletion of the floxed sequences in the Wt1-expressing cells (and their descendants). As Wt1 is expressed in the developing genitourinary system and in the mesothelia overlying most visceral organs, these mutant mice may be useful as fluorescent/Cre-lox tools for lineage-tracing/marking Wt1-expressing cells for studying cardiomyocytes in the developing heart.

If using these Wt1GFPCre mice with a Rosa-lacZ reporter allele, the donating investigator reports a preference for using the Gt(ROSA)26Sortm1Sho allele (reporting that other floxed alleles can be more broadly recombined by this Wt1GFPCre allele; perhaps due to transient, low level expression in the early embryo).

Of note, the Wt1GFPCre strain breeds poorly with non-productive matings and small litter sizes when it is too inbred. Therefore, every third generation, The Jackson Laboratory breeds heterozygous mice to B6129SF1/J hybrid mice (Stock No. <a href="https://www.jax.org/strain/101043">101043</a>) to maintain hybrid vigor. Wt1GFPCre mice exhibit whisker picking.

The Wt1GFPCre knock-in allele in these mice both abolishes Wilms tumor 1 homolog gene function and expresses an EGFPCre fusion protein directed to proepicardium and epicardium by the Wt1 promoter/enhancer elements. As Wt1 is expressed in the developing genitourinary system and in the mesothelia overlying most visceral organs, these mutant mice may be useful as fluorescent/Cre-lox tools for lineage-tracing/marking Wt1-expressing cells for studying cardiomyocytes in the developing heart.

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