Bromodomain-containing protein 4 (Brd4) contains two tandem bromodomains (BD1 and BD2) that bind preferentially to acetylated lysine residues found in histones and nonhistone proteins. This molecular recognition allows Brd4 to associate with acetylated chromatin throughout the cell cycle and regulates transcription at targeted loci. Recruitment of positive transcription elongation factor b, and possibly the general initiation cofactor Mediator as well, plays an important role in Brd4-regulated transcription. Selective contacts with acetyl-lysines in nucleosomal histones and chromatin-binding factors likely provide a molecular switch modulating the steps from chromatin targeting to transcriptional regulation, thus further expanding the 'acetylation code' for combinatorial regulation in eukaryotes.