Using CRISPR/Cas9 technology, a P2A-Flpo (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 into the targetted STOP codon of the Complexin-3 gene (Cplx3) on chromosome 9 in embryonic stem cells. This flpo coding sequence was further codon modified; standard flpo PCR primers may not anneal. Chimeric mice were generated by aggregating ES cells with morula-stage embryos and bred for germline transmission.The ampicillin cassette is still present in this allele. (J:101977)