A targeting vector was used to insert loxP sites on either side of exons 3 and 4, and a FRT site flanked neomycin resistance cassette. The construct was electroporated into C57BL/6-derived Bruce4 embryonic stem (ES) cells. Correctly targeted ES cells were injected into BALB/c recipient blastocysts. The donating investigator reported that all mice were maintained on a pure C57BL/6 background (see SNP note below). The resulting chimeric animals were crossed to C57BL/6 transgenic mice expressing Cre recombinase under the control of the cytomegalovirus promoter, to remove the floxed exons. The donating investigator reports that the mice do not carry the cre transgene.
A 32 SNP (single nucleotide polymorphism) panel analysis, with 27 markers covering all 19 chromosomes and the X chromosome, as well as 5 markers that distinguish between the C57BL/6J and C57BL/6N substrains, was performed on the rederived living colony at The Jackson Laboratory Repository. While the 27 markers throughout the genome suggested a C57BL/6 genetic background, 2 of 5 markers that determine C57BL/6J from C57BL/6N were found to be segregating. These data suggest the mice sent to The Jackson Laboratory Repository were on a mixed C57BL/6J ; C57BL/6N genetic background.
