Bromodexyuridine (BrdU) is a thymidine analog which is selectively incorporated into the DNA of proliferating cells to provide a marker for the DNA being replicated. The number of proliferating cells can then be detected in cell lysates, tissue sections or suspensions using an antibody specific for the BrdU. Previous methods of detecting DNA included the use of [3H]-thymidine which would be incorporated into the DNA and could then the DNA could be quantified by autoradiography or scintillation counting. These methods are more difficult and require more cleanup due to the radioactive material. An immunohistochemical assay provides a much simpler and cleaner method for detecting DNA in cells.