That cancer could be caused by viruses was discovered by Ellerman and Bang in 1908 when they showed that an avian leukaemia could be transmitted by cell-free extracts. During the past twenty years, the part played by viruses in the production of cancer has been the subject of intensive research and we now know a great deal about it. Several types of virus have been implicated. They are generally divided into the DNA viruses and the RNA viruses and then subdivided within these groups. All the tumour viruses have a rather simple structure in which the nucleic acid forms an internal core or nucleoid. This is surrounded by proteins which form a viral coat or envelope called the capsid. In most tumour viruses, it has a precise geometrical structure composed of protein sub-units called capsomeres. In some viruses the whole is encased in a lipoprotein envelope. The entire viral particle is sometimes referred to as a virion (figure 5.1).
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