transvection effects. These would include an indirect read- out of the expression patterns of the two transgenes (i.e. UAS- or LexAop-GFP) or a direct readout via antibody stain- ing or in situ hybridizations. Finally, our observation that UAS sequences can drive transcription in trans is also of interest given the widespread use of the GAL4/UAS system in Drosophila. There may be cases where activating a UAS-driven effector drives expres- sion not only of the effector, but also of a paired gene in trans. The chances of this phenomenon substantially affect- ing the outcome or interpretation of an experiment are prob- ably low, but worth considering. Acknowledgments We thank Robin Harris for ﬁrst reporting crosstalk between GAL4 and LexA transgenes at attP2; Bruce Baker, Carmen Robinett, and Troy Shirangi for valuable feedback and experimental suggestions; Barret Pfeiffer, Heather Dionne, and Gerry Rubin for DNA constructs and ﬂy strains; Elizabeth Gavis and Tzumin Lee for providing valuable ﬂy strains; Sean Eddy, Lee Henry, Carmen Robinett, Gerry Rubin, and members of the Truman lab for comments on the manuscript; Todd Laverty and the Janelia Farm Fly Facility for help managing ﬂy stocks; and Ting Wu and two anonymous reviewers for helpful and insightful comments that greatly improved the manuscript. Note added in proof: In this issue of GENETICS, Bateman and colleagues (pp. 1143–1155) report an independently developed, targeted transgene integration strategy for inves- tigating transvection. Their results agree with our own in several important respects. 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