The targeting construct inserted into the locus a CAG promoter, loxP site, neomycin resistance/STOP cassette, loxM site, and in an inverted cassette containing loxP and loxM sites, Tbx18 cDNA with a VP16 activator, IRES, Venus reporter sequence and polyadenylation sequence. Cre-mediated recombination is required to achieve expression of the inverted cassette.