Mi{FlpStop} is designed to allow conditional gene disruption, containing sequences that should disrupt gene expression if inserted in the correct orientation into an endogenous gene. The sequences consist of 1. transcription termination sequences (both SV40 and αTub84B), and 2. a splice acceptor sequence followed by stop codons in all three reading frames, which will result in translation termination if the construct is inserted into the codon intron in the correct orientation. This disruptive cassette is flanked by two pairs of target sites for FLP-mediated recombination (FRT and FRT3), which form a 'FLEx-switch' (design principles described in PMID:12665802), which means that the cassette can be inverted using FLP recombinase and then stably locked in the opposite orientation, generating the Mi{FlpStop.lock} construct. The inversion event also results in the generation of a functional UAS-driven reporter (Disc\RFP^{tdTom.UAS.FlpStop.lock}), by bringing the tdTomato fluorescent protein under the control of UAS regulatory sequences, providing a cell-autonomous readout of cassette inversion. Mi{FlpStop} is generated in vivo by using a donor construct containing the FlpStop cassette flanked by attB sites to replace the RMCE cassette present in an insertion of a Mi{MIC} element in a coding region of a gene of interest. The Mi{FlpStop} element generated in this way can be in either one of two possible orientations relative to the gene it is inserted into: 1. in the 'non-disrupting' (ND) orientation, the expression-disrupting sequences are on the non-transcribed strand and thus expression of the endogenous gene is not predicted to be affected, 2. in the 'disrupting' (D) orientation, the expression-disrupting sequences are on the transcribed strand and are thus predicted to affect expression of the endogenous gene. FLP-mediated recombination can be used to invert a 'non-disrupting' (ND) Mi{FlpStop} insertion into a Mi{FlpStop.lock} insertion in the 'disrupting locked' (Dlock) orientation or conversely, to invert a 'disrupting' (D) Mi{FlpStop} insertion into a Mi{FlpStop.lock} insertion in the 'non-disrupting locked' (NDlock) orientation.