A method of producing a transgenic cell comprising introducing into a cell a non-primate lentiviral expression vector comprising a nucleotide of interest (NOI). Also described is a method of producing a transgenic cell comprising introducing into a cell a lentiviral expression vector comprising a NOI capable of generating an antisense oligonucleotide, a ribozyme, an siRNA, a short hairpin RNA, a micro-RNA or a group 1 intron. Also described is a viral vector comprising a first nucleotide sequence, wherein said first nucleotide sequence comprises: (a) a second nucleotide sequence comprising an aptazyme; and (b) a third nucleotide sequence capable of generating a polynucleotide; wherein (a) and (b) are operably linked and wherein the aptazyme is activatable to cleave a transcript of the first nucleotide sequence such that said polynucleotide is generated.