Bromodeoxyuridine (5-bromo-2-deoxyuridine, BrdU) is a synthetic nucleoside which is an analogue of thymidine. BrdU is commonly used in the detection of proliferating cells in living tissues. BrdU can be incorporated into the newly synthesized DNA of replicating cells (during the S phase of the cell cycle), substituting for thymidine during DNA replication. Antibodies specific to BrdU can then be used to detect the incorporated chemical; thus indicating cells that were actively replicating their DNA. Binding of the antibody requires denaturation of the DNA by heat or acid.