sentences:  Like in HIV's replication cycle, these proteins are needed for viral budding from the host cell's plasma membrane, where the fully formed virions leave the cell to infect other cells.The Gag-v-Onc fusion protein from the Rous sarcoma virus illustrates the dual role that the fusion protein plays in b the viral and host cellular life cycle.For example, the viral gene Src (as in "sarcoma") is not necessary for viral reproduction, but does affect virulence.