UASp regulatory sequences drive expression of the entire osk coding sequence, which is joined to a modified bcd 3'UTR (the putative NRE sequence, a 45bp HpaI-EcoRV fragment has been replaced by a 10bp SpeI linker). An ATG triplet corresponding to the start codon for the osk long isoform has been mutated to CTG, so that the construct only expresses the short isoform of osk.