Breeding Protocols

This page contains recommended breeding strategies and protocols. If you have any questions, please get in touch with us at

Germline Transmission Testing (ES cell)

KOMP and EUCOMM Breeding Strategies (Recombinase)

GENSAT Breeding Recommendations

GENSAT BAC transgenic mouse lines should be utilized in the hemizygous state. Breeding of the BAC transgene to a homozygous state carries an increased risk of generating an unwanted phenotype for two reasons. First, the transgene inserts at random into the genome. Breeding the transgene to homozygosity results in mice that carry this insertion on both chromosomes, increasing the probability that the insertion will result in an observable phenotype. Second, BAC transgenes may have passenger genes also expressed from the BAC vector. Breeding to homozygosity doubles the number of these genes carried in the transgenic mouse, increasing the possibility that a phenotype will result from increased gene dosage. In our experience, observable phenotypes are rare in BAC transgenic mice when carried in the hemizygous state.

A reduced expression may concern some Cre and EGFP lines maintained in live colonies over time. There is a publication regarding variable expression for Cre lines, specifically a decrease over time, while still detecting it at the genomic level: There have also been studies that show GFP expression can deteriorate over time:

Mixed Background Strain Recommendations

For strains on a mixed background (such as mixed B6;129), we recommend using wildtype animals generated from heterozygous matings as controls. If homozygous mice are needed, we recommend maintaining your colony with heterozygous to heterozygous matings, as this will render both homozygous and wildtype mice.