This transgene consists of the CAG promoter (the cytomegalovirus immediate early (CMV-EI) enhancer followed by a 1.3-kb DNA segment including the promoter, first exon and first intron of the chicken beta-actin gene, with the 3' splice junction sequence replaced by that of the rabbit hemoglobin beta gene), a floxed chloramphenicol acetyltransferase (cat) gene with the SV40 small t intron and polyadenylation signal, and a full length human TBX3 cDNA with an in frame fusion to a hemagglutinin tag. Cre mediated recombination removes the cat gene allowing expression of TBX3.