Cis-acting

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A molecule may be described as cis-acting when it affects other entities only if they are physically adjacent. It may be considered 'the opposite' of a 'trans-acting' molecule.

A cis-acting region does not produce genes to regulate activity, but rather relies on the binding of other genes and enzymes to regulate whether adjacent genes on a particular strand of DNA are transcribed or not. The "operator" in the lac operon is an example of a cis-acting regulatory sequence, as it requires binding of lactose to begin expression of the rest of the gene.