Mi{FlpTag.smV5.2.lock} is a transgene generated in vivo by using FLP recombinase to stably invert the flip-excision (FLEx) switch present in Mi{FlpTag.smV5.2}. Mi{FlpTag.smV5.2} is designed to allow conditional gene tagging. It contains an artificial exon composed of a splice acceptor site, the smV5 tag and a splice donor site; this sequence can act as a protein trap element when inserted into a phase 2 coding intron of an endogenous gene in the correct orientation. The protein trap exon is flanked by two pairs of target sites for FLP-mediated recombination (FRT and FRT14), which form a FLEx switch (design principles described in PMID:12665802), which means that the protein trap exon can be inverted using FLP recombinase and then stably locked in the opposite orientation, generating the Mi{FlpTag.smV5.2.lock} transgene.