False colour transmission electron micrograph (TEM) of virions (virus particles) of a type C oncornavirus (retrovirus), a class of RNA virus associated with the formation of cancers in birds & mammals (including man). Individual virions are spherical, 100 nanometers in diameter and possess a lipoprotein membrane derived from the host cell. Retroviruses possess the enzyme reverse transcriptase, which permits the DNA translation of viral RNA and facilitates integration of viral and host cell genomes. The viral genome may then rest in a suppressed or inactive state within the host cell (a phenomenon known as latency). Magnification: X 35,000 (35mm).