5-Bromo-2-deoxyuridine (BrdU) is an analog of thymidine, and can be incorporated into the newly synthesized DNA of replicating cells (S-phase cells). BrdU is a commonly used in the detection of proliferating cells in living tissues. It works by substituting for thymidine during DNA replication and incorporating itself into the newly synthesized DNA (during S-phase). Antibodies specific for BrdU can then be used to detect the incorporated chemical, thus indicating cells that were actively replicating their DNA. Because BrdU can replace thymidine during the S phase of cell division, it can cause mutations.