Chromatin reader protein that recognizes and binds acetylated histones and plays a key role in transmission of epigenetic memory across cell divisions and transcription regulation. Remains associated with acetylated chromatin throughout the entire cell cycle and provides epigenetic memory for postmitotic G1 gene transcription by preserving acetylated chromatin status and maintaining high-order chromatin structure (PubMed:23589332, PubMed:23317504, PubMed:22334664). During interphase, plays a key role in regulating the transcription of signal-inducible genes by associating with the P-TEFb complex and recruiting it to promoters. Also recruits P-TEFb complex to distal enhancers, so called anti-pause enhancers in collaboration with JMJD6. BRD4 and JMJD6 are required to form the transcriptionally active P-TEFb complex by displacing negative regulators such as HEXIM1 and 7SKsnRNA complex from P-TEFb, thereby transforming it into an active form that can then phosphorylate the C-terminal domain (CTD) of RNA polymerase II (PubMed:23589332, PubMed:19596240, PubMed:16109377, PubMed:16109376, PubMed:24360279). Promotes phosphorylation of 'Ser-2' of the C-terminal domain (CTD) of RNA polymerase II (PubMed:23086925). According to a report, directly acts as an atypical protein kinase and mediates phosphorylation of 'Ser-2' of the C-terminal domain (CTD) of RNA polymerase II; these data however need additional evidences in vivo (PubMed:22509028). In addition to acetylated histones, also recognizes and binds acetylated RELA, leading to further recruitment of the P-TEFb complex and subsequent activation of NF-kappa-B (PubMed:19103749). Also acts as a regulator of p53/TP53-mediated transcription: following phosphorylation by CK2, recruited to p53/TP53 specific target promoters (PubMed:23317504). Acts as a chromatin insulator in the DNA damage response pathway. Inhibits DNA damage response signaling by recruiting the condensin-2 complex to acetylated histones, leading to chromatin structure remodeling, insulating the region from DNA damage response by limiting spreading of histone H2AX/H2A.x phosphorylation. Binds acetylated histone H4 (PubMed:29176719). Interacts with p53/TP53; the interaction is direct (PubMed:23317504). Interacts (via CTD region) with CDK9 and CCNT1, acting as an associated component of P-TEFb complex (PubMed:16109376, PubMed:16109377, PubMed:23317504, PubMed:24360279). Interacts with RELA (when acetylated at 'Lys-310')(PubMed:19103749). Interacts (via NET domain) with NSD3, CHD4, BICRA and ATAD5 (PubMed:21555454,PubMed:29176719). The interaction with BICRA bridges BRD4 to the GBAF complex (PubMed:29374058, PubMed:16109376, PubMed:16109377, PubMed:19103749, PubMed:21555454, PubMed:23317504). Interacts (via NET domain) with JMJD6 (via JmjC and N-terminal domains); the interaction is stronger in presence of ssRNA and recruits JMJD6 on distal enhancers (PubMed:24360279, PubMed:21555454, PubMed:29176719). Interacts with NSD3 (PubMed:29176719). Isoform B: interacts with NCAPD3 and SMC2 (PubMed:23728299). (Microbial infection) Interacts with bovine papillomavirus type 1 regulatory protein E2. This interactions may serve for the tethering of viral genomes to host mitotic chromosomes allowing successful partitioning of the viral genome during cell division. (Microbial infection) Interacts with Epstein-Barr virus (EBV) protein EBNA1; this interaction facilitates transcriptional activation by EBNA1. (Microbial infection) Interacts with human herpes virus-8 (HHV-8) protein LANA. Associates with acetylated chromatin (PubMed:21890894, PubMed:16109376). Released from chromatin upon deacetylation of histones that can be triggered by different signals such as activation of the JNK pathway or nocodazole treatment (PubMed:21890894, PubMed:16109376). Preferentially localizes to mitotic chromosomes, while it does not localizes to meiotic chromosomes (PubMed:21890894, PubMed:16109376). Ubiquitously expressed. The NET domain mediates interaction with a number of chromatin proteins involved in transcription regulation (NSD3, JMJD6, CHD4, GLTSCR1 and ATAD5). The C-terminal (CTD) region mediates interaction and recruitment of CDK9 and CCNT1 subunits of the P-TEFb complex (PubMed:16109376, PubMed:16109377). It is also required for maintenance of higher-order chromatin structure (PubMed:22334664). The 2 bromo domains mediate specific binding to acetylated histones via Asn-140 and Asn-433, respectively (PubMed:20871596). The exact combination of modified histone tails required to recruit BRD4 to target genes is still unclear. The first bromo domain has high affinity for acetylated histone H4 tail, whereas the second bromo domain recognizes multiply acetylated marks in histone H3 (PubMed:22464331). A number of specific inhibitors bind competitively to acetyl-lysine-binding residues Asn-140 and Asn-433, promoting removal from acetylated histones. Many of these inhibitors are benzodiazepine derivatives (PubMed:22137933, PubMed:22136404, PubMed:23517011, PubMed:23530754). Phosphorylation by CK2 disrupt the intramolecular binding between the bromo domain 2 and the NPS region and promotes binding between the NPS and the BID regions, leading to activate the protein and promote binding to acetylated histones. In absence of phosphorylation, BRD4 does not localize to p53/TP53 target gene promoters, phosphorylation promoting recruitment to p53/TP53 target promoters. A chromosomal aberration involving BRD4 is found in a rare, aggressive, and lethal carcinoma arising in midline organs of young people. Translocation t(15;19)(q14;p13) with NUTM1 which produces a BRD4-NUTM1 fusion protein. Some specific inhibitors of BRD4 that prevent binding to acetylated histones by binding Asn-140 and Asn-433 are promising therapeutic molecules for the treatment of leukemias. JQ1, a thieno-triazolo-1,4-diazepine derivative, and I-BET, a benzodiazepine derivative, have been tested on tumors with success (PubMed:20871596, PubMed:21068722, PubMed:21964340). Treatment with GSK1210151A (I-BET151, a I-BET derivative) has strong effets on mixed lineage leukemia and promotes myeloid differentiation and leukemia stem-cell depletion (PubMed:21964340). Does not contain the C-terminal (CTD) region required to recruit the P-TEFb complex. Extended N-terminus.