The use of EdU to label DNA in cells. (a) Structure of EdU. (b) Scheme of the click reaction for detecting EdU incorporated into cellular DNA. The terminal alkyne group, exposed in the major groove of the DNA helix, readily reacts with an organic azide (R can be any fluorophore, hapten, electron-dense particle, quantum dot, etc.) in the presence of catalytic amounts of Cu(I). Adapted with permission from Salic, A., Mitchison, T.J., 2008, A chemical method for fast and sensitive detection of DNA synthesis in vivo. Proc Natl Acad Sci U S A 105, 2415–2420, Copyright (2008) National Academy of Sciences U.S.A. [76].