The proteolytic events that cleave Gag and Gag-Pro-Pol are well characterized, but the event that triggers the protease is not well characterized. The PRGag, that is assembled in the immature virion weakly dimerizes, once PR is cleaved from the proprotein PR dimerizes and becomes an efficient protease. This assembly step may be part of the switch. Once the protease becomes active in the immature virion MA, CA, SP1, NC, SP2, P6, PR, RT, and IN are produced. This event, the production of these fragments would be the switch from immature to mature.