Figure 2.

Model of retroviral endogenization. Retroviral infection of a germ cell is thought to make integration into the host germ line (denoted by an asterisk) possible. If an infected germ cell develops into offspring, it will transmit its provirus to every single cell of the offspring, akin to inheritance of a host gene. In evolutionary time, germ-line integrated proviruses can either expand in number within the germ-line and in the population, ultimately achieving fixation, or become extinct by random events or selection pressure against them.