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Viruses can cause benign or malignant tumors in many species of animals (e.g., frogs, fishes, birds, and mammals). Despite the common occurrence of tumor viruses in animals, only a few viruses are associated with human tumors, and evidence that they are truly the causative agents exists for very few.

Tumor viruses have no characteristic size, shape, or chemical composition. Some are large, and some are small; some are enveloped while others are nonenveloped; some have DNA as their genetic material, and others have RNA. The factor that unites all of them is their common ability to cause cancer.

Tumor viruses are at the forefront of cancer research for two main reasons:

  1. They are more rapid, reliable, and efficient tumor producers than either chemicals or radiation. For example, many of these viruses can cause tumors in all susceptible animals in 1 or 2 weeks and can produce malignant transformation in cultured cells in just a few days.

  2. They have a small number of genes compared with a human cell (only three, four, or five for many retroviruses), and hence their role in the production of cancer can be readily analyzed and understood. To date, the genomes of many tumor viruses have been sequenced and the number of genes and their functions have been determined; all of this has provided important information.


The term malignant transformation refers to changes in the growth properties, shape, and other features of the tumor cell (Table 43–1). Malignant transformation can be induced by tumor viruses not only in animals but also in cultured cells. In culture, the following changes occur when cells become malignantly transformed.

TABLE 43–1Features of Malignant Transformation

Altered Morphology

Malignant cells lose their characteristic differentiated shape and appear rounded and more refractile when seen in a microscope. The rounding is due to the disaggregation of actin filaments, and the reduced adherence of the cell to the surface of the culture dish is the result of changes in the surface charge of the cell.

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