What is the C765-Gal4 driver?
C765 is a transgenic Drosophila strain that has a specific Gal4 expression pattern ("embryonic salivary glands and larval wing and leg discs"). 

Gal4 is a transcription factor from yeast which binds an upstream activating sequence (UAS) and also happens to function in other species (like flies). In the Gal4/UAS system, a strain expressing Gal4, under the control of a tissue specific promoter or a minimal promoter in an enhancer trap, is crossed with a strain expressing a gene of interest flanked at the 5' end by the UAS. The F1 generation will thus express the gene of interest tissue specifically (ie only in the tissue(s) expressing Gal4). 



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Further reading:

Phelps CB, Brand, AH. 1998. Ectopic Gene Expression in Drosophila Using Gal4 System. Methods 14(4):367-379.