Conversion of a bona fide retrovirus to an ''intracellularized'' element. (A) Electronic microscopy images of viral particles produced by IAPE, a ''classical'' endogenous retrovirus, budding at the cell membrane. (B) Images of viral particles produced by a modified IAPE (in which the N-terminal domain of Gag was changed). Particles are accumulating in the endoplasmic reticulum after budding into the cisternae (inset). Pm, Plasma membrane; Nu, Nucleus; ERm, Endoplasmic reticulum membrane. (C) Intracellular amplification efficiency of IAPE and modified IAPE.