The pM15 plasmid contains a recombination-mediated cassette exchange (RMCE) cassette flanked by a pair of inverted attP sites. This cassette contains a mutagenic gene trap element consisting of a splice acceptor site, stop codons in all three reading frames and an SV40 polyadenylation signal. A w^SH00017.3xP3 marker allele is present downstream of the gene trap element. pM15 is used to generate donor plasmids that can be used in combination with CRISPR/Cas9 to insert the attP-flanked RMCE cassette into a defined location in the genome by homology directed repair; the donor plasmid is generated by adding homology arms to each side of the pM15 RMCE cassette using the restriction sites in the plasmid.