In the field of molecular biology, cis-acting generally means "acting from the same molecule". It may be considered the opposite of trans-acting which generally means "acting from a different molecule".
In the context of transcription regulation, cis-acting elements are usually considered to be DNA sequences that, via transcription factors or other trans-acting elements or factors, regulate the expression of genes on the same chromosome. The "operator" in the lac operon is an example of a cis-acting regulatory sequence. This DNA sequence is bound by the lac repressor which in turn prevents transcription of the adjacent genes on the same DNA molecule. The lac operator is thus considered to "act in cis" on the regulation of the nearby genes. The operator itself doesn't code for any protein or RNA.
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