UAS regulatory sequences drive expression of a fusion protein containing both the Scer\FCY1 (a cytosine deaminase) and Scer\FUR1 (a uracil phosphoribosyltransferase) coding regions. The coding sequence has not been codon-optimized for Drosophila. The fusion protein is tagged at the N-terminal end with Tag:HA. This fusion protein is designed to be used in the 'EC-tagging' technique: when flies are exposed to 5-ethynylcytosine (EC), the sequential activity of the two enzymes encoded by the fusion protein converts the 5-ethynylcytosine into 5-ethynyluridine monophosphate that is subsequently incorporated into nascent RNAs. The ethynyl group allows detection and purification of the tagged RNAs.