Retrovirus vectors containing an internal attachment site: evidence that circles are not intermediates to murine retrovirus integration.

Murine cells were infected with a retrovirus vector containing a defective native attachment (att) site, an internal att site, and a neo gene. Analysis of the proviruses by virus rescue and Southern blots demonstrated that internal att sites were not utilized for integration and could not complement defects in the native site. These data suggest that murine retroviruses do not integrate in vivo through tandem long terminal repeat circular DNA intermediates.