The bromodomain and ET domain (BET) proteins belong to a group of bromodomain proteins and bind acetylated histones. Two of the currently known members of this protein family were implicated in transcriptional regulation. The two most studied BET proteins Brd2 and Brd4 have been shown to bind to viral proteins of herpesviruses and papillomaviruses. These pathogens often take advantage of the cellular function of the BET proteins and exploit it for their own purposes. In some cases though, viral proteins were shown to adapt BET proteins to new virus specific functions. Additionally some retroviruses seem to encode proteins that mimic Brd4 functions and hijack Brd4-associated protein complexes to use them for their own transcription.