UASt regulatory sequences drive expression of a mutated form of the 5' terminal part of E(spl)mγ-HLH, which contains a double point mutation in the basic domain (amino acid replacements K17N and E24Q). The E(spl)mγ-HLH is fused with sequence encoding the VP16 transcriptional activation domain, after E(spl)mγ-HLH residue 142.