The adenovirus type 5 E1A enhancer contains two functionally distinct domains: one is specific for E1A and the other modulates all early units in cis.

The adenovirus type 5 genome contains two distinct enhancer elements located at the left end of the viral chromosome. The first element is repeated and specifically regulates region E1A transcription within infected cells. One copy of this element is sufficient to fully activate E1A transcription in vivo. The second element is located between these repeated sequences and regulates transcription in cis of all early regions on the chromosome. These enhancer elements function independently of each other, and neither element is required for efficient viral DNA replication. Since mutations within the two E1A enhancer components generate different physiological responses, transcriptional enhancement can be achieved through multiple mechanisms.