Brd4 belongs to the BET family, a group of structurally related proteins containing two bromodomains. Through these two domains, Brd4 associates with mitotic chromosomes and its expression correlates with cell growth. Expression of Brd4 inhibits cell cycle progression from G(1) to S, due to binding to the largest subunit of replication factor C (RFC) to prevent DNA elongation. Altered Brd4 function correlates with poorly differentiated carcinoma, with aggresive phenotype and a highly lethal outcome.