Strategies for monitoring retrotransposition events with cellular resolution. (A) An overview of the LINE1 retrotransposition assay (adopted from (64)). The L1-cassette contains reporter (shown is GFP) in the opposite direction to normal L1 transcription. As transcription occurs, the intron that disrupts the reporter is removed by a splicing event. SD, splice donor; SA, splice acceptor; IVS, intervening sequence. (B) A schematic of gypsy-TRAP reporter (adopted from (84)). i) No integration: Expression of GAL80 under α-tubulin promoter with intact hot spots (ovo promoter) suppresses GAL4-mediated transcription, thus no GFP signal is detectable. ii) Gypsy integration: If integration of gypsy into the ovo binding site occurs, depicted by triangles, GAL4-driven GFP expression becomes detectable as GAL80 expression is ceased. UAS, Upstream Activating Sequence. (C) A gypsy-CLEVR reporter mimicking the replication cycle of retrovirus (adopted from (92)). During replication of gypsy retrotransposon, 5’ end of the 3’-LTR region (U3’’ in black box) and 3’ end of the 5’ LTR region (U5 in white box) are respectively used for synthesis of 5’ end of 5’-LTR region and 3’ end of 3’-LTR region. Note that both 5X UAS in 5’-LTR region (U5 in white box) and a reporter (GFP-P2A-mCherry) in 3’-LTR region (U3’’ in black box) are in opposite direction to gypsy transcription. Retrotransposition of gypsy-CLEVR leads 5X UAS in vicinity to the reporter (GFP-P2A-mCherry), allowing expression of the reporter by GAL4 activator. PBS, primer binding site; LTR, Long Terminal Repeats.