A cDNA encoding MerCreMer (cre recombinase fused to two modified estrogen receptor ligand binding domains at both ends), and an frt-flanked neomycin selection cassette were cloned in-frame with the Acbdg2 ATG start codon containing a second exon of Abcg2. Homologous recombination was performed in ES cells, and chimeras were bred to flp-expressing knock-in mice to delete the selection marker. (J:304949)