The minigene construct consists of the following fragments (5'-3'): A 5' BamHI-SacII 5.4 kb genomic fragment including part of Rx exon 1; a SacII (exon 1)-KpnI (exon 6) cDNA fragment; a 3' KpnI 6.7 Kb genomic fragment including part of Rx exon 6.