Mi{FlipFlop.0} is designed to allow conditional gene inactivation. It contains a 'FlipFlop' cassette composed of two independent modules oriented in opposite directions: 1. an intron phase 0 protein-trap (PT) module for tagging endogenous proteins with EGFP (this is generally non-disrupting), and 2: a gene-trap (GT) module designed to truncate the endogenous gene into which it inserts, as it contains T2A-mCherry followed by stop codons in all frames. Translation should skip at the T2A sequence, truncating the endogenous protein and producing a separate mCherry protein. The FlipFlop cassette can be inverted as it is flanked by two pairs of mutually incompatible FRT sites (FRT and FRT14) that form a flip-excision (FLEx) switch (PMID:12665802), which means that the cassette can be inverted using FLP recombinase and then stably locked in the opposite orientation. Mi{FlipFlop.0} is generated in vivo by using a donor construct containing the FlipFlop 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. Depending on the orientation of the cassette relative to the endogenous gene, the inserted Mi{FlipFlop.0} element can either be in the 'protein-trap' or 'gene-trap' orientation. FLPase mediated recombination can be used to invert the 'FlipFlop' cassette in vivo, producing Mi{FlipFlop.lock.0}. Inversion of a Mi{FlipFlop.0} insertion in protein-trap orientation will result in a Mi{FlipFlop.lock.0} insertion locked in gene-trap orientation, while inversion of a Mi{FlipFlop.0} insertion in gene-trap orientation will result in a Mi{FlipFlop.lock.0} insertion locked in protein-trap orientation.