A cassette encoding cre recombinase fused to 2 mutated murine estrogen receptor ligand-binding domains (Mer; Esr1*) followed by FRT flanked neo selection cassette are inserted into the start codon of Tnnt2 locus, with disruption of endogenous ATG by homologous recombination in ES cells. Flp-mediated recombination removed the selection cassette, leaving a single FRT site behind. (J:224086)