Construct: The nos promoter drives expression of bcd cDNA in which the 5' UTR and part of the 3'UTR have been replaced by 5' and 3'UTR sequences from nos. (Construction notes - 1. The 5'UTR also contains some sequence from the Xenopus leavis Β-globin 5'UTR originating in bcdTN3; 2. The bcd 3'UTR sequence present is truncated at an HpaI site 5' to the nos response element; 3. The nos 3'UTR fused to this HpaI site is an EcoRI-NotI fragment from nosgnosb and includes some nos 3' genomic sequence).