Tetracycline-repressible female lethal genetic systems for Lucilia cuprina. In the single component system , tTA expression is auto-regulated as the promoter contains 21 copies of the tTA binding site (tetO) upstream of the core promoter from the Lchsp70 gene . The Chtra intron is inserted within the tTA coding region. As a consequence, only female overexpress tTA due to sex-specific splicing of tTA transcripts. Binding of tTA to tetO results in activation (+++) of tTA gene expression. High female lethality was observed in most transgenic lines when reared on diet that lacks tetracycline . In the two-component system [41, 42], which is under development for L. cuprina, tTA expression is controlled by a promoter from a cellularization gene (e.g. bnk), which is most active in early embryos. tTA activates (+++) expression of hid, but only the female hid transcript encodes HID protein due to sex-specific splicing of the Chtra first intron.