Mi{FFL.2} is designed to allow conditional gene inactivation. It contains a 'FlipFloploxP' (FFL) cassette composed of two independent modules (protein trap and gene trap) oriented in opposite directions. The protein trap (PT) module is in intron phase 2 and is designed to tag endogenous proteins with EGFP; this is generally non-disrupting, but a pair of loxP sites has been added to flank each side of the protein trap module so that it can be removed using P1cre recombinase if the in-frame EGFP fusion disrupts normal protein function. The gene-trap (GT) module is 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 FFL 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{FFL.2} is generated in vivo by using a donor construct containing the FFL cassette flanked by attB sites to replace a RMCE cassette flanked by inverted attP sites (such as in the Mi{MIC} element) that is inserted in a coding region of a gene of interest. Depending on the orientation of the cassette relative to the endogenous gene, the inserted Mi{FFL.2} element can either be in the 'protein-trap' or 'gene-trap' orientation. FLP mediated recombination can then be used to invert the FFL cassette in vivo, producing a 'locked' insertion in the opposite trap orientation from that of the starting insertion.