A collection of conditional insertional alleles that contain a 'FlipFloploxP' (FFL) cassette; this allows gene function to be turned on or off using the flip-flop technique described in Nagarkar-Jaiswal et al., 2017, eLife 6: e26420 (FBrf0235931). The FFL cassette in this collection contains two independent modules (protein trap and gene trap) oriented in opposite directions. The protein trap (PT) module 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. Depending on the orientation in which the FFL cassette is inserted relative to the direction of transcription of an endogenous gene, the inserted element can either be in the 'protein-trap' or 'gene-trap' orientation for that gene. The inserted FFL cassette can subsequently be stably inverted in vivo using FLP recombinase (as it is flanked by two pairs of mutually incompatible FRT sites), producing a 'locked' insertion in the opposite trap orientation from that of the starting insertion.