The transgene consists of a mouse Prrx1 promoter and a Hand2 cDNA fused to the viral coactivator VP16 at the C-terminus. 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.