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Tobacco mosaic virus. Computer model showing the molecular structure of a tobacco mosaic virus (TMV) particle (virion). This plant virus is a tobamovirus, and has an RNA (ribonucleic acid) genome. It was first identified in 1886 in the tobacco plant (Nicotiana tabacum), one of the world's major cash crops. This virus also infects many other plant species. Symptoms are mosaic patterns on the leaves and severe crop loss. The virus is transmitted by seeds and contact between plants. (KEYSTONE/SCIENCE PHOTO LIBRARY/LAGUNA DESIGN)