ftz lacking the 'FIE3' instability element of the ftz 3' untranslated region expressed under the control of its endogenous promoter. A 0.7kb fragment from the rat aldolase B mRNA (nucleotides 856 to 1557 relative to the transcription start site) has been inserted into the ftz 3'UTR sequence, to allow the transcript produced by this construct to be distinguished from endogenous ftz transcripts by Northern blotting.