The transgene consists of a mouse Prrx1 promoter fused to a mutant Hand2 cDNA with a phenylalanine to proline substitution at residue 119 (F119P). The mouse Prrx1 promoter was used to drive transgene expression in the forelimb bud and anterior mesoderm of the hindlimb in E10.5 embryos. The Hand2 construct also contains an N-terminal Myc epitope tag and a 3' human growth hormone polyadenylation signal.