A Ubx minigene which contains all 4 Ubx exons (including the B element between the "a" and "b" 5' splice sites of the 5' common exon, E5') but whose introns have been shortened by deletion of internal sequences is expressed under the control of UASp regulatory sequences. Microexon mI has been mutated such that the protein coding potential of the transgene is maintained while the ultraconserved nucleotide sequence of mI has been disrupted by means of synonymous mutations. In addition, most of the homeobox has been replaced by a fragment encoding two Tag:HA tags.