The coding sequence in exon 1 from the translation start site was replaced by an inducible cre recombinase gene (cre/ERT2). Flp-mediated recombination removed the FRT-flanked neomycin resistance cassette inserted downstream of the recombinase. Immunohistochemistry confirmed expression in the atrial and ventricular endocardium, but not sinus venosus, at E8.5 and E9.5.