tetO regulatory sequences and an Hsp70 core promoter are fused upstream of sequence encoding the tTA driver. A 2974bp fragment that contains the complete female-specific intron from the Cochliomyia hominivorax tra gene has been inserted into the tTA coding sequence, immediately after the translation start codon. This is designed to result in expression of functional tTA protein only in females; if the female-specific intron is correctly spliced, the male transcript would include the male-specific tra exon, which contains several in-frame translation stop codons, while the female transcript would contain the full-length tTA open reading frame.