Construct: A dicistronic transgene consisting of the first cistron: cad 5' UTR upstream of the cad coding region fused to Ecol\lacZ. A 250 nucleotide fragment derived from exon 4 of the Antp leader sequence which contains the internal ribosome entry site (IRES) inserted between the coding regions. The second cistron is the hb promoter sequence, cad 5' UTR, ftz coding sequence and the cad 3' UTR.