EdU (5-ethynyl-2’-deoxyuridine) is a nucleoside, an analog of thymidine, carrying an ethynyl group. This nucleoside behaves as a substrate for cellular DNA replication machinery. After EdU incorporation, DNA containing ethynyl groups can be ligated by click chemistry reaction with various dye azides. Fluorescent DNA can be detected with microscopy, or cells can be sorted with FACS.