A method for producing viral vectors is described using packaging and producer cell lines is described. The producer cell comprises: (i) a first nucleotide sequence (NS) encoding a toxic viral envelope protein operably linked to a promoter; wherein the promoter is operably linked to at least one copy of a TRE; (ii) a second NS wherein the second NS comprises a sequence encoding a tetracycline modulator; (iii) a third NS encoding a retrovirus nucleocapsid protein; and (iv) a fourth NS comprising a retroviral sequence capable of being encapsidated in the nucleocapsid protein such that the retroviral vector particle titre obtainable from the producer cell is regulatable by tetracycline and an initial stimulus with sodium butyrate or functional analogues thereof.