An operon is a group of genes 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. They were first described by Franois Jacob and Jacques Monod in 1961.
The switch of an operon (that is, the "operator") is turned on unless a specific substance is bound to the operator. This substance is therefore called a repressor.