A 4kb region from upstream of the transcription start site of the PM isoform of Prm is fused upstream of the Ecol\lacZ coding region, which is fused upstream of exons 5, 6 and 7 of Prm. In addition, an artificial intron (intron 8 of Prm with the middle portion of the intron removed) is also present followed by the SV40 polyadenylation signal.