A
viral vector production
system is provided which
system comprises: (i) a viral
genome comprising at least one first
nucleotide sequence encoding a
gene product capable of binding to and effecting the cleavage, directly or indirectly, of a second
nucleotide sequence, or transcription product thereof, encoding a viral polypeptide required for the
assembly of viral particles, (ii) a third
nucleotide sequence encoding said viral polypeptide required for the
assembly of the viral
genome into viral particles, which third nucleotide sequence has a different nucleotide sequence to the second nucleotide sequence such that said third nucleotide sequence, or transcription product thereof, is resistant to cleavage directed by said
gene product; wherein at least one of the
gene products is an external guide sequence capable of binding to and effecting the cleavage by
RNase P of the second nucleotide sequence. The
viral vector production
system may be used to produce viral particles for use in treating or preventing
viral infection.