BET (bromodomains and extra-terminal) domain is a highly conserved motif composed of 110 amino acids. It is composed of four anti-parallel α-helices, and contains a bromodomain in the amino-terminal region and an extraterminal (ET) protein-protein interaction domain in the carboxy-terminal region. The two conserved loops are linked with a hydrophobic cleft. This domain is present in BRD (bromodomain-containing) proteins, such as BRD2, BRD3 and BRD4, and these proteins are thus known as BET family proteins. Humans are thought to contain 56 bromodomains present in 42 proteins.

BRD (bromodomain) proteins are known as chromatin “readers”, as they recruit chromatin-regulating enzymes which are responsible for histone modification. These proteins eventually act on promoters and control gene expression.