Electron microscopic analysis of the oricoHERV-K113_GagProPol VLP morphology. (A) Immature particles budding from the cell and being released. (B) Particles with condensed cores can be observed close to the cell membrane demonstrating an active protease and the ability of the VLPs to mature. (C) Thin section micrograph of a pellet made by ultracentrifugation of supernatants from VLP-producing cells. All VLPs show condensed cores. (D) Transmission electron microscopy of VLPs showing incorporation of a reconstituted HERV-K113 envelope protein  expressed in trans. The arrow indicates the Env proteins on the surface of the virion. (E) Scanning electron microscopy of VLPs at the surface of cells. The upper panel shows VLPs produced with pcDNAoricoHERV-K_GagProPol and the lower panel VLPs with reconstituted Env at the surface (arrows) which was expressed in trans.