Figure 3: Models of retroviral structure. (Models courtesy of Lou Henderson.) (A) The immature virion. The Gag and Gag-Pol proteins are shown with different colors to suggest the domains corresponding to the mature proteins formed from these precursors. The SU and TM components of Env are shown jutting out from the lipid membrane, as are selected host proteins that may be incorporated in some retroviruses. See text for details. (B) The mature virion. The major Gag and Pol proteins are shown for HIV as an example. Retroviruses of this genus are distinguished by the characteristic conical shape of their cores. See text for details.