Abstract: This invention provides a mammalian cell useful for retroviral packaging comprising two plasmids, both of which comprise the 5′ long terminal repeat (LTR) sequence from a helper virus, neither of which comprise a functional &psgr; packaging sequence or a 3′ LTR from the helper virus, one of which comprises the env gene from the helper virus and the other of which comprises the gag and pol genes from the helper virus. This invention also provides a process for preparing a producer cell useful for transferring a foreign gene into a mammalian cell which comprises treating the above-described mammalian cell with a vector plasmid so as to insert the vector plasmid into the cell and thus create the producer cell, the vector plasmid comprising the foreign gene, a functional &psgr; packaging sequence from the helper virus, both the 5′ and 3′ LTRS from the helper virus, and a gene encoding a selectable or identifiable phenotypic trait, and recovering the producer cell so created.

Abstract: The invention relates to a mammalian cell line useful for retroviral packaging comprising two plasmids, the first such plasmid comprising in 5' to 3' order: a DNA sequence comprising a 5' long terminal repeat (LTR); a mutated .psi. packaging sequence; a DNA sequence comprising the encoding portion of the Moloney murine leukemia virus (MULV) env gene; and the second such plasmid comprising in 5' to 3' order: a DNA sequence comprising a 5' long terminal repeat (LTR); a mutated .psi. packaging sequence; a DNA sequence comprising the encoding portion of the Moloney murine leukemia virus (MULV) gag and pol genes; a selectable marker; and an origin of replication.The invention also relates to processes for preparing a producer cell line useful for transferring a gene of interest into recipient mammalian cells in vitro.