The transgene comprises the node-specific enhancer (NDE), which in the mouse genome resides in a 0.8-kb region 9.5-8.7 kb upstream of the Nodal coding sequence, followed by nucleotides -664 through +113 of the mouse heat shock protein 1B genomic sequence, a fusion gene combining the coding sequences of the E. coli genes gpt, TrpS and lacZ, and an SV40 polyadenylation signal. (J:83358)