2.5kb fragment containing an Hsp70 promoter, ATG and 940bp coding sequence are fused in frame to a fragment of the P-element (nucleotides 1935 to 2183) which includes part of ORF2, the third intron and part of ORF3, followed by lacZ reporter sequence and an SV40 polyadenylation site. The intron sequence must be spliced out in order for functional lacZ protein to be produced.