In the field of molecular biology, trans-acting generally means “acting from a different molecule”. It may be considered the opposite of cis-acting which generally means “acting from the same molecule”.

In the context of transcription regulation, a trans-acting element is usually a DNA sequence that contains a gene. This gene codes for a protein (or microRNA or other diffusible molecule) that will be used in the regulation of another target gene.^[1] The trans-acting gene may be on the same chromosome as the target gene, but the activity is via the intermediary protein or RNA that it encodes. Cis-acting elements, on the other hand, do not code for protein or RNA. Both the trans-acting gene and the protein/RNA that it encodes are said to “act in trans” on the target gene.