A targeting vector consisting of a cassette containing ERT2-Cre-recombinase genes with frt-PGK-neomycin (neo)-bpA-frt was used for homologous recombination in ES cells using the CRISPR-Cas9 system to replace the exon 1 of the Sfrp5 gene. Frt-neo was then removed by crossing with Rosa-Flp mice.