The Rosa26-LSL-Cas9 targeting vector used was designed with (from 5' to 3') a 5' homology arm, a ubiquitously expressed CAG promoter, loxP-flanked 3xSV40 polyA stop cassette, a 3X-FLAG epitope tag, a mammalian codon-optimized cas9 gene (derived from Streptococcus pyogenes CRISPR associated protein 9 [SpCas9]) flanked by two nuclear localization signals, a P2A ribosomal skip cleavage peptide sequence, an enhanced Green Fluorescent Protein (EGFP) gene, a woodchuck hepatitis virus post-transcriptional regulatory element, bovine growth hormone polyA, pPGK-Neo-pA positive selection cassette and a 3' homology arm. The construct was electroporated into 129S-derived R1 embryonic stem (ES) cells. Correctly targeted ES cells were injected into C57BL/6N blastocysts. The resulting chimeric animals were tested for germline transmission by crossing to C57BL/6N mice. The mice were then backcrossed to C57BL/6N. Upon arrival at The Jackson Laboratory, the mice were crossed to C57BL/6J (Stock No. 000664) at least once to establish the colony. The mice were then backcrossed to FVB/NJ (Stock No. 001800) using a marker assisted protocol.
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. One of the 43 markers, on Chromosome 9, was segregating with 129.
