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An operon is a group of key nucleotide sequences including an operator, a common promoter, and one or more structural genes that are controlled as a unit to produce messenger RNA (mRNA). Operons occur primarily in prokaryotes and nematodes. Although located independent of the operon gene, a "Regulator" gene is present which codes for the production of a repressor or corepressor protein. The location and condition of the regulator, promoter, operator and structural DNA sequences can determine the effects of common mutations. They were first described by François Jacob and Jacques Monod in 1961.