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In an organism, be it plant, animal or human, almost every gene has its own promoter sequence, which is typified as a DNA stretch that controls how a gene is expressed in a cell. Hence, the activity of a promoter controls in which cell type, during which developmental stage or during what environmental condition a certain gene is expressed. However, the most widely used promoter in plant biotechnology is actually not derived from a plant, but a pathogenic virus. How and why did that happen? Here's a short history of the CaMV 35S promoter.
Corrected some minor mistakes, updated the format to match the other 'Short histories' and added a new citation.