The transcriptional unit comprises the expressed exons and intervening introns, as well as 5' non-coding sequences required for RNA polymerase binding (Shine-Delgarno sequence) and trailing 3' non-coding sequences. Outside the transcriptional area, the 5' flanking region ("upstream") gene region includes the 'CAT box' and 'TATA box' promoters required for RNA polymerase recognition prior to transcription. Enhancers that regulate occurrence, timing, and amount of transcription occur in both the upstream region and the 3' flanking region ("downstream") region. Multiple enhancers may occur many hundreds of nucleotides upstream.

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