Several researchers have suggested that the PR-mediated initial cleavage occurs via an intramolecular mechanism [37, 38, 39, 40, 41] . Although we did not search for the presence of a PR dimer, PR was theoretically capable of being released from the chimera as a fully processed dimer, since the cleavage site at the PR/LZ junction remained intact. Compared to the wt LZ fusion, the placement of a mutant LZ (Kz) at either the N-or Cterminus of the wt LZ (Wz) resulted in reduced Gag cleavage efficiency (Fig. 2) . This may be attributable to the inability of WKz or KWz to promote Gag-PR multimerization (Fig. 4E) .