UAS regulatory sequences drive expression of a reporter in which two coding sequences (rLUC and GFP-tagged pLUC) are separated by a kel gene test fragment that extends from 84 nucleotides 5' to 6 nucleotides 3' relative to the UGA kel ORF1 stop codon and in which the UGA codon has been mutated to an in-frame UGG. Designed to be used a control in an assay to test the ability of the kel fragment to mediate stop codon readthrough; readthrough efficiency can be calculated from the ratio of GFP produced by P{UAS-rLUC-UGA-6-fLUC::GFP} relative to this control construct.