The @[email protected] by which the synthesis of an enzyme is prevented by @[email protected] molecules. In many cases, the end product of a synthesis chain (e.g. an amino acid) acts as a feed-back corepressor by combining with an intracellular aporepressor protein, so that this complex is able to @[email protected] the function of an operator. As a result, the whole @[email protected] is prevented from being transcribed into mRNA, and the expression of all @[email protected] necessary for the synthesis of the end product amino acid is abolished.