There is provided a plasmid system for transfection into a cell to create a producer cell, the system comprising: a. a helper plasmid comprising a first nucleotide sequence encoding Murine leukemia virus (MLV)-derived Gag and Pol poly-proteins; b. an envelope plasmid comprising a second nucleotide sequence encoding an Env protein; c. a genome plasmid comprising a third nucleotide sequence encoding a retroviral genome, wherein the first nucleotide sequence is codon-shuffled to remove any significant regions of homology with the third nucleotide sequence; and wherein the second nucleotide sequence is codon-optimised for expression in the producer cell.