A targeting vector containing an Enhanced Green Fluorescent Protein (EGFP, Clontech) cDNA sequence, loxP-flanked neomycin resistance gene, herpes simplex virus thymidine kinase gene, and SV40 polyadenylation site sequence was used to disrupt the first 390 bp of exon 2. The construct was electroporated into 129P2/OlaHsd derived E14.1 embryonic stem (ES) cells which were transiently transfected with a Cre recombinase vector to remove the selection cassette. ES cells that had successfully undergone Cre-mediated recombination (removing the loxP-flanked neo cassette and leaving a single loxP site downstream of EGFP) were injected into recipient blastocysts. The donating investigator reported that resulting chimeric animals were backcrossed to C57BL/6 (see SNP note below) for ten generations before being made homozygous. During the backcross, mice were likely bred to a B6.CD45.1 congenic strain (harboring the CD45.1 (Ly5.1 or Ptprca) allele rather than the CD45.2 (Ly5.2 or Ptprcb) allele normally present in C57BL/6 mice). These mice, however, were bred such that they still harbor the expected CD45.2 (Ly5.2 or Ptprcb) allele normally present in C57BL/6 mice.
In 2020, a 48 SNP (single nucleotide polymorphism) panel analysis, with 43 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. Three of the 5 markers that determine C57BL/6J from C57BL/6N were found to be segregating, including the rd8 recessive mutation associated with retinal degeneration on Chromosome 1 within the Crb1 locus. These data suggest the mice sent to The Jackson Laboratory Repository were on a mixed C57BL/6N;C57BL/6J genetic background.
