An enhancer trap transgenic approach was used to generate these mice. The TK-bgi-icre-bGHpA transgene was designed with a thymidine kinase minimal promoter, a beta-globin intron, an optimized variant of Cre recombinase (iCre; improved with mammalian codon usage, removed putative cryptic splice sites, altered stop codon, and reduced CpG content to limit the chances of epigenetic silencing in mammals), and a bovine growth hormone polyA signal. This transgene was injected into C57BL/6J embryos. Founder mice (F0) from line 1555 were bred to "C57 R26R" mutant mice from the Baylor College of Medicine (Gt(ROSA)26Sortm1Sor mice; similar to, but perhaps not as backcrossed as Stock No. 003474). The donating investigator reports that Cre-positive transgenic pups were bred to C57BL/6 mice or "C57 R26R" (see SNP note below) to maintain the colony. Upon arrival, transgenic mice were selectively bred to C57BL/6J inbred mice (Stock No. 000664) to remove the Gt(ROSA)26Sortm1Sor mutation.
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, 1 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.
