A targeting vector containing iCre (codon-improved Cre recombinase), a polyadenylation sequence, and an FRT site flanked PGK-NEO cassette was inserted upstream of the initiation codon site of exon 2 of the the Esr1 gene of a BAC clone (ID RP23-7D5). Flp-mediated recombination removed the FRT-flanked neo cassette. (J:250442)