Application 5-Bromo-2′-deoxyuridine 5′-triphosphate (5-BrdUTP) may be used as a labeling TdT substrate for terminal deoxynucleotidyl transferase (TdT) reactions or for DNA synthesis. BrdUTP is also used as an effective mutagenic agent. 5-BrdUTP labeling may be detected by immunological techniques or 5-BrdUTP may be derivitized by biotinylation or other methods for detection by fluorogenic or chromogenic methods.