A hybrid dual construct system suitable for expressing the coding sequence of ABCA4 gene in a host cell, comprising: a) a first polynucleotide comprising in a 5'-3' direction: - a 5'-inverted terminal repeat (5'-ITR) sequence; - a promoter sequence of the human rhodopsin kinase (RK); - the 5' end portion of said coding sequence of the ABCA4 gene, said 5' end portion being operably linked to and under control of said promoter; - the 5' end portion of a sequence of a synthetic intron comprising a nucleic acid sequence of a splicing donor (SD) signal (SEQ ID NO: 1); - a nucleic acid sequence of a recombinogenic region derived or originating from alkaline phosphatase (AP); and - a 3'-inverted terminal repeat (3'-ITR) sequence; and b) a second polynucleotide comprising in a 5'-3' direction: - a 5'-inverted terminal repeat (5'-ITR) sequence; - a nucleic acid sequence of a recombinogenic region derived or originating from alkaline phosphatase (AP); - the 3' end portion of a sequence of a synthetic intron comprising a branch site, a polypyrimidine tract and a splicing acceptor (SA) signal (SEQ ID NO: 2); - the 3' end of said coding sequence of the ABCA4 gene; - a poly-adenylation (pA) signal nucleic acid sequence; and - a 3'-inverted terminal repeat (3'-ITR) sequence. a) a first polynucleotide comprising in a 5'-3' direction: - a 5'-inverted terminal repeat (5'-ITR) sequence; - a promoter sequence of the human rhodopsin kinase (RK); - the 5' end portion of said coding sequence of the ABCA4 gene, said 5' end portion being operably linked to and under control of said promoter; - the 5' end portion of a sequence of a synthetic intron comprising a nucleic acid sequence of a splicing donor (SD) signal (SEQ ID NO: 1); - a nucleic acid sequence of a recombinogenic region derived or originating from alkaline phosphatase (AP); and - a 3'-inverted terminal repeat (3'-ITR) sequence; and - a 5'-inverted terminal repeat (5'-ITR) sequence; - a promoter sequence of the human rhodopsin kinase (RK); - the 5' end portion of said coding sequence of the ABCA4 gene, said 5' end portion being operably linked to and under control of said promoter; - the 5' end portion of a sequence of a synthetic intron comprising a nucleic acid sequence of a splicing donor (SD) signal (SEQ ID NO: 1); - a nucleic acid sequence of a recombinogenic region derived or originating from alkaline phosphatase (AP); and - a 3'-inverted terminal repeat (3'-ITR) sequence; b) a second polynucleotide comprising in a 5'-3' direction: - a 5'-inverted terminal repeat (5'-ITR) sequence; - a nucleic acid sequence of a recombinogenic region derived or originating from alkaline phosphatase (AP); - the 3' end portion of a sequence of a synthetic intron comprising a branch site, a polypyrimidine tract and a splicing acceptor (SA) signal (SEQ ID NO: 2); - the 3' end of said coding sequence of the ABCA4 gene; - a poly-adenylation (pA) signal nucleic acid sequence; and - a 3'-inverted terminal repeat (3'-ITR) sequence. - a 5'-inverted terminal repeat (5'-ITR) sequence; - a nucleic acid sequence of a recombinogenic region derived or originating from alkaline phosphatase (AP); - the 3' end portion of a sequence of a synthetic intron comprising a branch site, a polypyrimidine tract and a splicing acceptor (SA) signal (SEQ ID NO: 2); - the 3' end of said coding sequence of the ABCA4 gene; - a poly-adenylation (pA) signal nucleic acid sequence; and - a 3'-inverted terminal repeat (3'-ITR) sequence. <EOS>