How PR is activated to mediate virus maturation is not completely clear. One proposal is that interaction among Pr160 gagpol molecules triggers the activation of embedded PR, which in homodimeric form mediates Gag and Gag-Pol cleavage following PR autocleavage from Pr160 gag-pol . Maintenance of the Pr55 gag / Pr160 gag-pol expression ratio is critical to virus assembly; the artificial overexpression of Pr160 gag-pol or PR drastically reduces virion production as a result of enhanced Gag processing by overexpressed PR activity [14, 15, 16, 17, 18, 19, 20] . Equally important is the Pr160 gag-pol sequence and structure, since sequence mutations upstream or downstream of PR often result in defective virus maturation or Gag cleavage [4, 21, 22, 23, 24] . Impaired Gag cleavage is assumed as being due, at least in part, to impaired PR activation, which is likely secondary to inadequate PR dimer interaction. Since natural RT is heterodimeric [25, 26] , there is speculation that RT in the Gag-Pol context facilitates Pr160 gag-pol -Pr160 gag-pol interaction via RT-RT interaction, which in turn influences PR activation. Consistent with this scenario, RT deletion mutations can lead to severely impaired PR-mediated Gag processing [23] . In addition, efavirenz (EFV), a nonnucleoside reverse transcriptase inhibitor that enhances RT dimerization in vitro [27, 28] , reduces virus production as a result of greatly enhanced Gag and Gag-Pol cleavage [29, 30] . Furthermore, a single amino acid substitution in RT (W402A) leads to significantly reduced virus production due to markedly enhanced PR-mediated Gag cleavage [31] . Combined, these data suggest that the RT domain plays an important role in PR activation by influencing PR dimer interaction.