The Esr1-cre targeting vector was designed to insert an attP site, a 2A oligopeptide (mediates ribosomal skipping), the Cre recombinase gene, a frt-flanked PGKneo-polyA cassette, and an attP site within the 3' UTR of the estrogen receptor 1 alpha gene (Esr1). The targeting construct was electroporated into 129S6/SvEvTac-derived TC-1 embryonic stem (ES) cells. Chimeric males were bred with 129S6/SvEvTac females to establish the colony. Heterozygotes were bred to ACT-FLPe mice to remove the frt-flanked PGKneo-polyA cassette. Mice were bred to remove the ACT-FLPe transgene from the background. (J:207677)