The next question is why premature PR-mediated Gag and Gag-Pol cleavage do not occur during virus assembly, given that multiple assembly domains outside of protease promote Gag-Pol multimerization. One possibility is that the transframe peptide p6 pol may play a role in modulating PR dimer interface interaction, thus preventing premature PR activation. Due to a blocking mutation at the p6 pol /PR cleavage, p6 pol -retaining PR is defective in mediating virus maturation [42, 43, 44, 45] , suggesting that a fully functional PR requires the removal of p6 pol . However, to our knowledge there are no reports on the role of p6 pol in PR dimer interaction in a Gag-Pol context. We found that a Gag-Pol mutant with deleted p6 pol was incapable of efficiently processing coexpressed Pr55 gag , which argues against the possibility of p6 pol playing a role in suppressing PR activation [46] . According to one recent study, the insertion of a larger reporter sequence in the p6 pol region markedly impairs virus maturation, whereas partial substitution with a heterologous sequence does not [47] . This suggests that structural conformation, rather than a specific sequence upstream of PR, is important for determining PR activation.