UAS regulatory sequences drive expression of a chimeric protein in which the Pvr extracellular ligand-binding domain has been replaced by the dimerization domain of the bacteriophage λ cI repressor (to induce constitutive dimerization of the encoded protein). A Tag:MYC epitope has been inserted at the junction of the λ and Pvr sequences. In addition, 9 Tyr residues in the Pvr intracellular domain have been mutated to non-phosphorylatable Phe residues (amino acid replacements Y985F, Y998F, Y1008F, Y1046F, Y1048F, Y1264F, Y1271F, Y1278F and Y1286F).