Ethidium Bromide (Homidium bromide, EtBr, EB) is an intercalating agent which resembles a DNA base pair and commonly used as a fluorescent tag (nucleic acid stain) in molecular biology laboratories for techniques such as agarose gel electrophoresis.
Ethidium Bromide (EtBr) is sometimes added to running buffer during the separation of DNA fragments by agarose gel electrophoresis. It is used because upon binding of the molecule to the DNA and illumination with a UV light source, the DNA banding pattern can be visualized. The mode of binding of EtBr is intercalation between the base pairs. This binding changes the charge, weight, conformation, and flexibility of the DNA molecule. The mobility of DNA was always less in the gels with EtBr[1].
[1] Sigmon J, et al. Electrophoresis. 1996, 17(10):1524-7.