P{FRT(24xMS2-SL)} is a transgenic donor construct containing a recombination-mediated cassette exchange (RMCE) cassette flanked by FRT sites. The RMCE cassette consists of two inverted attB sites which surround a '24xMS2-SL' stem-loop cassette that contains 24 copies of Tag:MS2 (the cassette contains 12 tandem repeats of a sequence encoding 2 unique Tag:MS2 stem loops separated by a short linker, and the version of the stem loops used were originally optimized for stable expression in bacteria). A w^+mW.hs marker is located outside of the RMCE cassette, between one of the attB sites and one of the FRT sites. Upon expression of FLP recombinase, the RMCE cassette is released as a circular DNA, which in the presence of phiC31:int integrase can undergo RMCE with a target site containing inverted attP sites, replacing the target 'inverted attP' cassette with the 24xMS2-SL stem-loop cassette.