A minigene comprising a 573bp fragment from directly upstream of the predicted transcription start site of Syp promoter 4, the 223bp Syp-RD/Syp-RV 5'UTR, EYFP, cDNA encoding the Syp-PV coding sequence (2037bp) and an 804bp fragment comprising the Syp-RT/Syp-RV 3'UTR plus some 3' genomic sequence.