Using CRISPR/Cas9 technology, the P2A-FlpO sequence (encoding a viral 2A oligopeptide that mediates ribosomal skipping and a codon-optimized FLPe gene modified for translation in mammalian cells and higher recombination efficiency in cells and in mice) was inserted after the STOP codon of the nephronectin gene (Npnt) on chromosome 3 in v6.5 embryonic stem (ES) cells. Chimeric mice were generated by aggregating ES cells with morula-stage embryos.