Figure 1

Electron microscopical analysis of viral contaminants. Viral particles from the supernatant of 293T cells were pelleted through a 20% sucrose cushion. Ultrathin sections of the fixed pellet show two types of particles resembling retroviruses. The more abundant one (*) exhibits a central electron-dense core, while the core of the other type of particles (→) is located excentrically. Scale bar: 100 nm.