Using CRISPR/Cas9 endonuclease-mediated genome editing, a donor plasmid designed to insert a codon improved cre recombinase (iCre) and a polyadenylation sequence immediately downstream of the start codon of the prolactin family 8, subfamily a, member 2 (Prl8a2) gene, a single guide RNA (ACTAGCAGTCTTATCCAACCNGG), and the cas9 nuclease with a puromycin resistance sequence were transfected into G4 embryonic stem (ES) cells. (J:326357)