The MMLV retroviral gene expression vector system is a highly efficient system that stably integrates foreign genes into mammalian cell genomes, and is also a particularly popular gene transduction method in iPS cell preparation.

The MMLV retroviral vector is derived from Moloni (A's) murine leukemia virus and belongs to the retroviral family, and the wild-type retroviral genome is a linear double plus-stranded RNA.

The MMLV retroviral recombinant vector construct was transfected into packaging cells together with the helper plasmid. In packaging cells, DNA fragments located between two long terminal repeats (LTRs) are transcribed into RNA and packaged into viral particles by viral proteins expressed by helper plasmids. The packaged live virus will be released into the supernatant, which can directly collect or further concentrate the virus for transfected target cells.

When viruses transduce target cells, the viral RNA released into the host cell is reverse transcribed into DNA with the help of reverse transcriptase, which is then randomly integrated into the genome of the host cell. In the viral vector, the DNA fragments located in both LTR and the viral genome are stably integrated into the genome of the target cell. 
 
For more information on retroviral gene expression vectors, please refer to the following literature.
 

Reference Documentation	Theme
Exp Hematol. 31:1007 (2003)	Review
J Virol. 61:1639 (1987)	Extended packaging signal increases the titer of MMLV vectors
Gene Ther. 7:1063 (2000)	Tropism of MMLV vectors depends on packaging cell lines
Nat Protoc. 6:346 (2011)	Tropism of MMLV vectors depends on packaging plasmids