BrdU (5-bromo-2-deoxyuridine) is a thymidine analogue which is incorporated into the cells of DNA synthetic phase. Replicating cells undergo DNA synthesis in a highly regulated, S-phase of the cell cycle. The regulation of cell proliferation is central to tissue morphogenesis during the development of multicellular organisms. Furthermore, loss of control of cell proliferation underlies the pathology of diseases like cancer.