A sequence cassette encoding T2A-QF2 (with an Hsp70 terminator sequence) followed by a loxP cassette containing a Disc\RFPDsRed.3xP3.cUa marker has been inserted into the endogenous mex1 locus. The cassette was inserted using the CRISPR/Cas9 technique together with a donor plasmid to drive homology directed repair. The sgRNA sequence used to target the gene was: CGTATGCTGCGCTTCACAAT.