The BAC clone 283O2 covers 241 kb around Esr1 genomic sequence (5’, 169 kb; 3’, 72 kb), which contains exon1–4 out of the Esr1 gene. For BAC recombination cassette, 393-bp of 5’ flanking region and 148-bp of 3’ flanking region were used as homology arms. By homologous recombination in bacteria, the ZsGreen cDNA was introduced into translation start site of the Esr1 gene in the BAC clone.