Since cell samples were collected between 48 and 72 h posttransfection, it is possible that Gag processing reached a level of stability that prevented us from detecting any differences in efficiency between the wt form and mutants. To test this possibility, and to confirm the effect of LZ domain placement on Gag processing efficiency, we collected samples at 24 and 48 h following the transient expression of wt and mutants. We observed that both PRWz and PRWWz showed significantly higher cellular p24 gag /Pr55 gag ratios compared to those of wt or Gag/PR (Fig. 2B) .