A turning point in cancer research: sequencing the human genome.

been made by using model systems of limited complexity, such as cancer cells in vitro and oncogenic viruses. The use of cell cultures avoided the complexity of the whole animal but not the complexity of the animal genome. The use of oncogenic viruses seemed to circumvent this complexity by replacing it with the extraordinary simplicity of the viral genome. This simplicity made the study of viruses very productive. The persistence of the transformed state in a cell clone could be explained by the persistence of the viral genome in cells (1); genetic and molecular results showed that transformation is the consequence of the expression ofone or a few viral genes. Finally, the viral transforming genes, or "oncogenes," and the proteins they specify were identified. The crowning development was the demonstration that in retroviruses the onco-