Color enhanced transmission electron micrograph of Murine leukemia virus particles. Animal viruses have capsids (protein case) with polyhedral or helical symmetry; the genetic material is DNA or RNA. After fusing with the host cell, the viral DNA or RNA enters the host, instructing the nucleus to begin replicating the virus. When the host cell lyses, many virus particles are released for further infection. Leukemia is a malignant neoplasm of blood-forming tissues. Magnification = 160,000x.