Morphological Transformation of Human Astrocytes by Visna Virus with Complete Virus Production

VISNA, a medium-sized RNA virus, is comparable in mode of maturation and a number of other properties with the RNA tumour viruses^1,2. The virion also carries RNA-dependent DNA polymerase^3–5. This enzyme was first discovered in RNA tumour viruses and thought perhaps unique to them^6–9; but it was subsequently found in a very few other RNA viruses^3,10, where tumorigenicity was not a known attribute. The question whether RNA-dependent DNA polymerase is a hallmark of malignancy therefore remains entirely open.