The pM14 plasmid consists of a recombination-mediated cassette exchange (RMCE) cassette flanked by a pair of inverted attP sites. Inside the attP sites is a cassette flanked by a pair of direct FRT sites. This in turn 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 Avic\GFP^3xP3.cLa marker allele followed by a polyadenylation signal is present downstream of the gene trap element. pM14 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 pM14 RMCE cassette using the Type IIS restriction sites in plasmid.