DNA synthesis labeling with EdU

EdU labeling uses a modified nucleoside, 5-ethynyl-2'-deoxyuridine, incorporated into newly synthesized DNA, then chemically couples it to a fluorescent reporter.

A 2008 experiment injected adult mice with $100\mu \mathrm{g}$ of this label and inspected intestinal sections 96 hours later. Fixed sections on slides received $10\mu \mathrm{M}$ Alexa568-azide for 10 minutes. Imaging with a fluorescence microscope combined the reporter image with a Hoechst DNA counterstain and a transmitted-light view. Labeled cell nuclei marked descendants of biological cells synthesizing DNA during exposure, rather than identifying current S phase at collection. ^[ref]

The same study's cultured-cell controls compared overnight $10\mu \mathrm{M}$ label, no label, and label with $10\mathrm{mM}$ hydroxyurea. No-label samples had little reporter background; hydroxyurea strongly suppressed incorporation. The camera detected residual signal with a 3-second exposure versus 40 milliseconds under otherwise identical settings, demonstrating why acquisition settings matter. These controls support synthesis-dependent labeling, without proving that every labeled biological cell completes division. ^[ref]