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-RD coding sequence (2124bp) and an 811bp fragment comprising the Syp-RD 3'UTR plus some 3' genomic sequence.