A new virus in a spontaneous mammary tumor of a rhesus monkey.

This study investigated the ultrastructure and development of virus particles detected in a spontaneous mammary tumor of an 8 year old rhesus monkey. During a 3-month period of tumor growth, several biopsies were performed. Thin section electron microscopy of the tumor tissue revealed 2 types of particles: 1) an intracytoplasmic, electron-dense, ring-shaped particle, and 2) an extracellular particle with an outer unit membrane and a central dense nucleoid. The intracytoplasmic development and virus maturation by a process of budding at the level of the cell membrane was reconstructed.