The transcriptional unit comprises the expressed exons and intervening introns, as well as 5' and 3' non-coding sequences required for RNA polymerase binding. The 5' flanking region ("upstream") region includes the promoters required for RNA polymerase recognition, as well as enhancers that regulate timing and amount of transcription. Enhancers also occur in the 3' flanking region ("downstream") region.

    By convention, the upstream downstream orientation corresponds to the 5' 3' direction of the DNA sense (coding) strand, and is drawn left to right.