PQR is a genetically encoded tag that allows for equimolar separation of an upstream protein of interest and a downstream protein of interest, all contained within a single strand of RNA. When one of those proteins is a fluorescent reporter, the number of fluorescent molecules produced is stoichiometric with the number of molecules of the protein of interest produced, and thus the fluorescence output can be used as a readout for the number of molecules of interest, i.e., its relative protein concentration. The PQR sequence is derived from single-stranded RNA virus sequences encoding peptides called cis-acting hydrolase elements (CHYSELs). CHYSELs can interact with the ribosome during protein translation to produce non-canonical protein coding events and separate a nascent polypeptide chain from an actively translating sequence, thereby allowing multiple proteins to be produced from a single, polycistronic RNA strand. The mechanism by which separation occurs is due to the specific sequence of CHYSEL residues upstream of a glycine proline separation point, which causes the ribosome to stall and inhibits the peptidyl transferase activity, thereby forcing the peptide chain to be released - the ribosome skips the glycyl-prolyl peptide bond and re-initiates from the proline, and translation continues with the downstream protein. The CHYSEL sequence used in Drosophila cells is EGRGSLLTCGDVEENPGP, taken from the Thosea asigna insect virus (T2A). It was modified and then codon de-optimized at specific residues (which decreases translation speed and enhances reliable protein separation) to result in the following DNA sequence for the Drosophila PQR (with glycine and serine linker, codon variation 2): 5'-GGAAGCGGAGAAGGT CGTGGTAGTCTACTAACGTGTGGTGACGTCGAGGAAAATCCTGGACCT-3'. (FBrf0233565.)