A method of producing a replication defective retrovirus comprising transfecting a producer cell with the following: iii) a retroviral genome; iv) a nucleotide sequence coding for retroviral gag and pol proteins; and iii) nucleotide sequences encoding other essential viral packaging components not encoded by the nucleotide sequence of (ii); characterised in that the nucleotide sequence coding for retroviral gag and pol proteins is codon optimised for expression in the producer cell.
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