A cassette that encodes the P2A peptide and EGFP has been inserted at the C-terminal end of the open reading frame of the endogenous Rx locus. The P2A peptide promotes ribosomal skipping, resulting in the separate translation of Rx and EGFP proteins from the bicistronic mRNA produced by the modified locus. In addition, a Disc\RFPDsRed.3xP3.cUa marker is present downstream of the Rx 3'UTR sequence.