A targeting vector was designed to replace exon 1 of the transcription factor 21,Tcf21 (capsulin; or epicardin) with a mercremer protein followed by a frt-flanked neo cassette. cre recombinase cDNA sequence flanked on each end with a mutated murine estrogen receptor (mer) ligand-binding domain. Flp-mediated recombination deletes the neo cassette.