A novel agent for fast and sensitive detection of DNA synthesis in vivo. 5-EdU is incorporated into proliferating cells followed by detection with a fluorescent azide using click ligation. In contrast to the use of BrdU, the new method does not require sample fixation or DNA denaturation. May be used for studying cell proliferation in the central nervous system and may be combined with BrdU staining in a novel protocol allowing for double labeling of DNA synthesis. EdU incorporation is rapidly becoming the gold standard method for DNA labeling.