A method of expressing a functional protein in an insect via sex-specific alternative splicing of an RNA transcript, the method comprising: a) transcribing, in said insect, a heterologous polynucleotide from an expression system to provide an RNA transcript, the expression system comprising: a promoter operably linked to a heterologous polynucleotide sequence encoding the functional protein, defined between a start codon and a stop codon; and an intronic splice control sequence comprising a protein binding domain; wherein the protein binding domain comprises the DNA consensus sequence shown in SEQ ID NO:1 or its RNA equivalent, wherein the intronic splice control sequence comprises a splice donor sequence GT on its 5' end (5'-GT), and wherein the intronic splice control sequence is flanked by a 5' guanine nucleotide; and wherein the intronic splice control sequence is 3' to the ATG start codon of the heterologous polynucleotide sequence; b) alternatively splicing the RNA transcript of the heterologous polynucleotide sequence, in co-operation with a spliceosome, to yield a first spliced messenger RNA (mRNA) product, which does not comprise a continuous open reading frame extending from the start codon to the stop codon, and a further alternative spliced mRNA product which comprises a continuous open reading frame extending from the start codon to the stop codon, defining said functional protein for expression. a) transcribing, in said insect, a heterologous polynucleotide from an expression system to provide an RNA transcript, the expression system comprising: a promoter operably linked to a heterologous polynucleotide sequence encoding the functional protein, defined between a start codon and a stop codon; and an intronic splice control sequence comprising a protein binding domain; wherein the protein binding domain comprises the DNA consensus sequence shown in SEQ ID NO:1 or its RNA equivalent, wherein the intronic splice control sequence comprises a splice donor sequence GT on its 5' end (5'-GT), and wherein the intronic splice control sequence is flanked by a 5' guanine nucleotide; and wherein the intronic splice control sequence is 3' to the ATG start codon of the heterologous polynucleotide sequence; a promoter operably linked to a heterologous polynucleotide sequence encoding the functional protein, defined between a start codon and a stop codon; and an intronic splice control sequence comprising a protein binding domain; wherein the protein binding domain comprises the DNA consensus sequence shown in SEQ ID NO:1 or its RNA equivalent, wherein the intronic splice control sequence comprises a splice donor sequence GT on its 5' end (5'-GT), and wherein the intronic splice control sequence is flanked by a 5' guanine nucleotide; and wherein the intronic splice control sequence is 3' to the ATG start codon of the heterologous polynucleotide sequence; b) alternatively splicing the RNA transcript of the heterologous polynucleotide sequence, in co-operation with a spliceosome, to yield a first spliced messenger RNA (mRNA) product, which does not comprise a continuous open reading frame extending from the start codon to the stop codon, and a further alternative spliced mRNA product which comprises a continuous open reading frame extending from the start codon to the stop codon, defining said functional protein for expression.