Abstract

Transcriptional enhancer sequences have been shown to play a pivotal role in the regulation of some highly expressed genes. First described in eukaryotic viruses, the discovery of enhancers has augmented the previously defined control‐sequence motifs to give a more complete understanding of eukaryotic transcriptional regulatory mechanisms. Some properties of enhancers that distinguish them from other regulatory sequences include their ability to function in a position‐ and orientation‐independent manner. Furthermore, the observation that some enhancers and transcriptional promoters exhibit tissue specificity in their activity has generated considerable interest. Enhancer activation is thought to involve sequence‐specific DNA‐binding proteins but the process by which this leads to an increase in the initiation of transcription remains obscure. Although viral enhancers are more fully characterized, several cellular enhancers have recently been identified. One such enhancer sequence has been detected in a 3′ flanking region of a highly expressed human placental lactogen gene. This enhancer can increase the transcriptional rate of an adjacent gene by at least 50‐fold.