Also known as 5-bromo-2-deoxyuridine, this chemical is a base analog of thymidine, with the thymine substituted by bromouracil. Because BrdU can replace thymidine, excessive exposure to the chemical can cause mutations.

BrdU is a common chemical used in the detection of proliferating cells in living tissues. It works by substituting for thymidine during DNA replication (being a base analog of thymidine) and incorporating itself into the newly synthesized DNA. Antibodies specific for BrdU (linked to a fluorescent molecule) can then be used to detect the incorporated chemical, thus indicating cells that were actively replicating its DNA (proliferating).