Forced integration of Moloney murine leukemia virus DNA with a mutant integration site occurs through recombination with VL30 DNA.

The integration of retroviral DNA to form the provirus requires the presence of short inverted repeat sequences at the termini of the linear viral DNA. We have previously described the construction of a mutant of Moloney murine leukemia virus, carrying an 8-bp deletion in the U5 terminus, that is defective in establishing the integrated provirus. We have now used a selectable marker to allow recovery of rare proviruses formed after transduction from NIH/3T3 cells by retroviral vectors carrying this mutation. Analysis of two such proviruses showed that both were recombinants between the vector and VL30 DNA and arose such that the VL30 sequences could provide an intact terminal sequence for integration.